Cathode active material for lithium ion secondary battery, and process for its production

ABSTRACT

To provide a cathode active material for a lithium ion secondary battery excellent in the cycle characteristics and rate characteristics even when charging is conducted at a high voltage. A cathode active material for a lithium ion secondary battery, which comprises particles (III) having a covering layer comprising a metal oxide (I) containing at least one metal element selected from the group consisting of elements in Groups 3 and 13 of the periodic table and lanthanoid elements, and a compound (II) containing Li and P, on the surface of a lithium-containing composite oxide comprising lithium and a transition metal element, wherein the atomic ratio of said P to said metal element (P/metal element) contained within 5 nm of the surface layer of the particles (III) is from 0.03 to 0.45.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/230,950 filed Mar. 31, 2014, which is in turn acontinuation of PCT Application No. PCT/JP2012/075410, filed on Oct. 1,2012, which is based upon and claims the benefit of priority fromJapanese Patent Application No. 2011-217358 filed on Sep. 30, 2011. Thecontents of those applications are incorporated herein by reference inits entirety.

TECHNICAL FIELD

The present invention relates to a cathode active material for a lithiumion secondary battery and a process for its production. Further, thepresent invention relates to a cathode for a lithium ion secondarybattery, and a lithium ion secondary battery, employing the cathodeactive material for a lithium ion secondary battery.

BACKGROUND ART

In recent years, lithium ion secondary batteries are widely used forportable electronic instruments such as cell phones or notebook-sizepersonal computers. As a cathode active material for a lithium ionsecondary battery, a composite oxide of lithium with a transition metal,etc. (hereinafter referred to also as a lithium-containing compositeoxide), such as LiCoO₂, LiNiO₂, LiNi_(0.8)Co_(0.2)O₂ or LiMn₂O₄, isemployed.

Further, for a lithium ion secondary battery for portable electronicinstruments or vehicles, it is desired to reduce the size and weight,and it is desired to further improve the discharge capacity per unitmass or such characteristics that the discharge capacity will notsubstantially decrease after repeating the charge and discharge cycle(hereinafter referred to also as cycle characteristics). Further,particularly in its application to vehicles, it is desired to furtherimprove such characteristics that the discharge capacity will notdecrease when discharging is conducted at a high discharge rate(hereinafter referred to also as rate characteristics). As a method forimproving such cycle characteristics and rate characteristics, it hasbeen known to be effective to provide a covering layer on the surface oflithium-containing composite oxide particles.

Patent Document 1 discloses a method of forming a treated surface layercontaining a compound represented by a chemical formula of MXOk on thesurface of an active material for a lithium ion secondary battery. Here,in the formula, M is at least one member selected from the groupconsisting of Na, K, Mg, Ca, Sr, Ni, Co, Si, Ti, B, Al, Sn, Mn, Cr, Fe,V, Zr and combinations thereof, X is an element selected from the groupconsisting of P, S, W and combinations thereof, and k is a number withina range of from 2 to 4.

Further, Patent Document 2 discloses a cathode active material for alithium ion secondary battery having a lithium compound such as Li₂SO₄or Li₃PO₄ impregnated on the surface of a lithium-containing compositeoxide. By the presence of such a lithium compound on the surface of thecathode active material, it functions as a physical barrier, wherebydissolution, into an electrolytic solution, of manganese ions in thelithium-containing composite oxide can be suppressed, and byincorporating a compound containing a bivalent metal atom (such as ZnO),it is possible to increase the valency of manganese atoms in thevicinity of the surface of the lithium-containing composite oxide,whereby elution of manganese ions can be further suppressed.

Patent Document 3 discloses a cathode active material for a lithium ionsecondary battery, which comprises a cathode active material capable ofabsorbing/desorbing lithium, a lithium phosphate compound and Al₂O₃. Bymixing the lithium phosphate compound and Al₂O₃, it is possible toimprove the lithium ion conductivity and at the same time to obtain goodthermal stability and a high discharge capacity and to obtain goodcharge and discharge cycles.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent No. 4,582,990

Patent Document 2: JP-A-2006-73482

Patent Document 3: JP-A-2008-71569

DISCLOSURE OF INVENTION Technical Problems

However, in the method disclosed in Patent Document 1, it was necessaryto dry a large amount of water to form the treated surface layer, andthus, the method had a problem such that not only a large energy fordrying was required, but also the cathode active material was likely toagglomerate during the drying, to form coarse particles. Further,although a treated surface layer made of AlPO₄ is disclosed, it wasdifficult to obtain an active material for a lithium ion secondarybattery excellent in the cycle characteristics and rate characteristics.

Whereas, in the method disclosed in Patent Document 2, in addition tothe lithium-containing composite oxide, a lithium compound is added, andthus, the alkali tends to be excessive. Since the alkali functions as acatalyst to decompose a carbonate as the solvent in the electrolyticsolution, there was a problem that it caused generation of a gas.Further, even if a compound having a bivalent metal atom was impregnatedon the cathode active material, it was not possible to obtain sufficientcycle characteristics.

Likewise, in the method disclosed in Patent Document 3, in addition tothe cathode active material, a lithium compound is added, and thus, thealkali tends to be excessive. Further, a solid and a liquid wereseparated by means of a centrifugal separator after the surfacetreatment, whereby a large amount of water was required to be treated bywaste water treatment, and there was a problem such that the cathodeactive material was likely to be agglomerated to form coarse particlesat the time of drying.

The present invention has been made to overcome the above problems, andit is an object of the present invention to provide a cathode activematerial for a lithium ion secondary battery excellent in the cyclecharacteristics and rate characteristics even when charging is conductedat a high voltage, and a process for producing a cathode active materialfor a lithium ion secondary battery to obtain such a cathode activematerial, as well as a cathode for a lithium ion secondary battery, anda lithium ion secondary battery, employing such a cathode activematerial for a lithium ion secondary battery.

Solution to Problems

The present invention provides a cathode active material for a lithiumion secondary battery, a cathode for a lithium ion secondary battery, alithium ion secondary battery, and a process for producing a cathodeactive material for a lithium ion secondary battery, havingconstructions of the following [1] to [13].

[1] A cathode active material for a lithium ion secondary battery, whichcomprises particles (III) having a covering layer comprising a metaloxide (I) containing at least one metal element selected from the groupconsisting of elements in Groups 3 and 13 of the periodic table andlanthanoid elements, and a compound (II) containing Li and P, on thesurface of a lithium-containing composite oxide comprising lithium and atransition metal element, wherein the atomic ratio of said P to saidmetal element (P/metal element) contained within 5 nm of the surfacelayer of the particles (III) is from 0.03 to 0.45.[2] The cathode active material for a lithium ion secondary batteryaccording to the above [1], wherein said metal element is at least onemetal element selected from the group consisting of Al, Y, Ga, In, La,Pr, Nd, Gd, Dy, Er and Yb.[3] The cathode active material for a lithium ion secondary batteryaccording to the above [1] or [2], wherein the compound (II) is Li₃PO₄.[4] The cathode active material for a lithium ion secondary batteryaccording to any one of the above [1] to [3], wherein the atomic ratioof said P to said metal element (P/metal element) contained within 5 nmof the surface layer of the particles (III) is from 0.10 to 0.40.[5] The cathode active material for a lithium ion secondary batteryaccording to any one of the above [1] to [4], wherein the value of themolar ratio of the metal element to the lithium-containing compositeoxide is from 0.001 to 0.03.[6] A cathode for a lithium ion secondary battery comprising the cathodeactive material for a lithium ion secondary battery as defined in anyone of the above [1] to [5], and a binder.[7] A lithium ion secondary battery comprising the cathode as defined inthe above [6], an anode and a non-aqueous electrolyte.[8] A process for producing a cathode active material for a lithium ionsecondary battery, which comprises:

a first contact step of contacting a powder of a lithium-containingcomposite oxide comprising lithium and a transition metal element, and afirst aqueous solution which contains a cation having at least one metalelement selected from the group consisting of elements in Groups 3 and13 of the periodic table and lanthanoid elements,

a second contact step of contacting said powder of thelithium-containing composite oxide, and a second aqueous solution whichcontains an anion having P and which does not contain a cation havingthe metal element, and

a heating step of heating, after the first and second contact steps, theobtained treated powder of the lithium-containing composite oxide to atemperature of from 250 to 700° C., wherein

in the entire aqueous solution having the first and second aqueoussolutions put together, |(number of moles of said anion contained insaid second aqueous solution×valency of said anion)/(number of moles ofsaid cation contained in said first aqueous solution× valency of saidcation) is less than 1.

[9] The process for producing a cathode active material for a lithiumion secondary battery according to the above [8], wherein the firstcontact step and the second contact step are separate steps, and thefirst contact step is conducted after the second contact step.[10] The process for producing a cathode active material for a lithiumion secondary battery according to the above [8] or [9], wherein thefirst aqueous solution contains at least one member selected from thegroup consisting of Al³⁺, Y³⁺, Ga³⁺, In³⁺, La³⁺, Pr³⁺, Nd³⁺, Gd³⁺, Dy³⁺,Er³⁺ and Yb³⁺, and the second aqueous solution contains PO₄ ³⁻.[11] The process for producing a cathode active material for a lithiumion secondary battery according to any one of the above [8] to [10],wherein the solvent in the first aqueous solution and the second aqueoussolution is water only.[12] The process for producing a cathode active material for a lithiumion secondary battery according to any one of the above [8] to [11],wherein at least one of the first contact step and the second contactstep is conducted by adding and mixing the first aqueous solution or thesecond aqueous solution to the powder of the lithium-containingcomposite oxide.[13] The process for producing a cathode active material for a lithiumion secondary battery according to any one of the above [8] to [11],wherein at least one of the first contact step and the second contactstep is conducted by applying the first aqueous solution or the secondaqueous solution by spray coating to the powder of thelithium-containing composite oxide.

Advantageous Effects of Invention

According to the present invention, it is possible to obtain a cathodeactive material for a lithium ion secondary battery excellent in thecycle characteristics and rate characteristics, even when charging isconducted at a high voltage.

Further, according to the process of the present invention, it ispossible to produce with good productivity, a cathode active materialfor a lithium ion secondary battery excellent in the cyclecharacteristics and rate characteristics, even when charging isconducted at a high voltage.

Still further, according to the cathode for a lithium ion secondarybattery of the present invention, and the lithium ion secondary batteryemploying the cathode, it is possible to realize excellent cyclecharacteristics and rate characteristics, even when charging isconducted at a high voltage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a chart showing the results of measurement of XRD (X-raydiffraction) with respect to the cathode active materials obtained inExamples 1 and 2 and Comparative Examples 1 and 2.

FIG. 2 is a chart showing the results (Al_(2p)) of measurement by an XPS(X-ray photoelectron spectroscopic) analysis with respect to the cathodeactive materials obtained in Examples 1 and 2.

FIG. 3 is a chart showing the results (P_(2p)) of measurement by an XPSanalysis with respect to the cathode active materials obtained inExamples 1 and 2.

DESCRIPTION OF EMBODIMENTS

[Cathode Active Material for Lithium Ion Secondary Battery]

The cathode active material for a lithium ion secondary battery of thepresent invention comprises particles (III) made of a lithium-containingcomposite oxide comprising lithium and a transition metal element, andhaving a covering layer formed on the surface thereof. The coveringlayer comprises a metal oxide (I) containing at least one metal elementselected from the group consisting of elements in Groups 3 and 13 of theperiodic table and lanthanoid elements, and a compound (II) containingLi and P. And, in the particles (III), the atomic ratio of elements(P/metal element) contained within 5 nm of the surface layer of theparticles (III) is from 0.03 to 0.45. Here, the numerator of the atomicratio is P, and the metal element as the denominator is a metal elementselected from the group consisting of elements in Groups 3 and 13 of theperiodic table and lanthanoid elements, and does not contain Li.

Here, in this specification, “the periodic table” is meant for a(long-form) periodic table (Groups 1 to 18).

Now, the lithium-containing composite oxide to constitute a cathodeactive material of the present invention, the covering layer and thecathode active material made of the particles (III) having the coveringlayer formed on the surface of lithium-containing composite oxideparticles, will be described as follows.

<Lithium-Containing Composite Oxide>

The lithium-containing composite oxide in the present inventioncomprises lithium and a transition metal element. The transition metalelement may, for example, be at least one member selected from the groupconsisting of Ni, Co, Mn, Fe, Cr, V and Cu.

The lithium-containing composite oxide is, for example, preferably acompound (i) represented by the following formula (1), a compound (ii)represented by the following formula (2-1), or a compound (iii)represented by the following formula (3). One of these materials may beused alone, or two or more of them may be used in combination. From sucha viewpoint that the discharge capacity is a high capacity, the compound(ii) is particularly preferred, and the compound represented by thefollowing formula (2-2) is most preferred.

(Compound (i))

The compound (i) is a compound represented by the following formula (1).Li_(a)(Ni_(x)Mn_(y)Co_(z))Me_(b)O₂  (1)In the formula (1), Me is at least one member selected from the groupconsisting of Mg, Ca, Sr, Ba and Al. Further, 0.95≤a≤1.1, 0≤x≤1, 0≤y≤1,0≤z≤1, 0≤b≤0.3, and 0.90≤x+y+z+b≤1.05.

Examples of the compound (i) represented by the formula (1) includeLiCoO₂, LiNiO₂, LiMnO₂, LiMn_(0.5)Ni_(0.5)O₂,LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂, LiNi_(0.85)Co_(0.10)Al_(0.05)O₂ andLiNi_(1/3)Co_(1/3)Mn_(1/3)O₂.

(Compound (ii))

The compound (ii) is a compound represented by the following formula(2-1). The representation of the compound represented by the formula(2-1) is a composition formula prior to conducting treatment such ascharge and discharge, or activation. Here, the activation is meant forremoval of lithium oxide (Li₂O), or lithium and lithium oxide, from thelithium-containing composite oxide. As a usual activation method, anelectrochemical activation method may be mentioned wherein charging isconducted at a voltage larger than 4.4 V or 4.6 V (a value representedas a potential difference from the oxidation-reduction potential ofLi⁺/Li). Further, an activation method may be mentioned which isconducted chemically by carrying out a chemical reaction employing anacid such as sulfuric acid, hydrochloric acid or nitric acid.Li(Li_(x)Mn_(y)Me′_(z))O_(p)F_(q)  (2-1)In the formula (2-1), Me′ is at least one member selected from the groupconsisting of Co, Ni, Cr, Fe, Al, Ti, Zr and Mg. Further, in the formula(2-1), 0.09<x<0.3, y>0, z>0, 1.9<p<2.1, 0≤q≤0.1 and 0.4≤y/(y+z)≤0.8,x+y+z=1, and 1.2<(1+x)/(y+z). That is, in the compound represented bythe formula (2-1), the proportion of Li exceeds 1.2 times by mol to thetotal of Mn and Me′. Further, the formula (2-1) is characterized also inthat it represents a compound containing a specific amount of Mn, andthe proportion of Mn to the total amount of Mn and Me′ is preferablyfrom 0.4 to 0.8, more preferably from 0.55 to 0.75. When Mn is withinsuch a range, the discharge capacity becomes to be a high capacity.Here, q represents the proportion of F, but when F is not present, q is0. Further, p is a value determined depending upon x, y, z and q and isfrom 1.9 to 2.1.

In a case where the lithium-containing composite oxide is a compoundrepresented by the formula (2-1), the composition ratio of Li element tothe total molar amount of the transition metal elements is preferably1.25≤(1+x)/(y+z)≤1.75, more preferably 1.35≤(1+x)/(y+z)≤1.65,particularly preferably 1.40≤(1+x)/(y+z)≤1.55. When this compositionratio is within such a range, it is possible to obtain a cathodematerial having a high discharge capacity per unit mass, when a highcharging voltage of at least 4.6V is applied.

As the compound (ii), a compound represented by the following formula(2-2) is more preferred.Li(Li_(x)Mn_(y)Ni_(v)Co_(w))O_(p)  (2-2)In the formula (2-2), 0.09<x<0.3, 0.36<y<0.73, 0<v<0.32, 0<w<0.32,1.9<p<2.1, and x+y+v+w=1.

In the formula (2-2), the composition ratio of Li element to the totalof Mn, Ni and Co elements is 1.2<(1+x)/(y+v+w)<1.8, preferably1.35<(1+x)/(y+v+w)<1.65, more preferably 1.45<(1+x)/(y+v+w)<1.55.

As the compound (ii), particularly preferred isLi(Li_(0.16)Ni_(0.17)Co_(0.08)Mn_(0.59))O₂,Li(Li_(0.17)Ni_(0.17)Co_(0.17)Mn_(0.49))O₂,Li(Li_(0.17)Ni_(0.21)Co_(0.03)Mn_(0.54))O₂,Li(Li_(0.17)Ni_(0.14)Co_(0.14)Mn_(0.55))O₂,Li(Li_(0.18)Ni_(0.12)Co_(0.12)Mn_(0.58))O₂,Li(Li_(0.18)Ni_(0.16)Co_(0.12)Mn_(0.54))O₂,Li(Li_(0.20)Ni_(0.12)Co_(0.08)Mn_(0.60))O₂,Li(Li_(0.20)Ni_(0.16)Co_(0.08)Mn_(0.56))O₂ orLi(Li_(0.20)Ni_(0.13)Co_(0.113)Mn_(0.54))O₂.

The compounds represented by the above formulae (2-1) and (2-2) arepreferably ones taking a layered rock salt type crystal structure (spacegroup R-3m). Further, the ratio of the Li element to the transitionmetal element is high, whereby in the XRD (X-ray diffraction)measurement using CuKα ray as the X-ray source, a peak is observedwithin a range of 2θ=20 to 25° like layered Li₂MnO₃.

(Compound (iii))

The compound (iii) is a compound represented by the following formula(3).Li(Mn_(2-x)Me″_(x))O₄  (3)In the formula (3), Me″ is at least one member selected from the groupconsisting of Co, Ni, Fe, Ti, Cr, Mg, Ba, Nb, Ag and Al. 0≤x<2. Thecompound (iii) may, for example, be LiMn₂C₄, LiMn_(1.5)Ni_(0.5)O₄,LiMn_(1.0)Co_(1.0)O₄, LiMn_(1.85)Al_(0.15)O₄ or LiMn_(1.9)Mg_(0.1)O₄.

The lithium-containing composite oxide in the present invention is inthe form of particles. The shape of the particles is not particularlylimited and may be spherical, needle-form, plate-form or the like.However, it is preferably spherical, since it is thereby possible toincrease a filling ability. Further, a plurality of such particles maybe agglomerated to form secondary particles, and also in such a case,spherical secondary particles are preferred, which are capable ofincreasing the filling ability. In the present invention, the averageparticle size (D50) means a volume-based accumulative 50% size which isa particle size at a point of 50% on an accumulative curve when theaccumulative curve is drawn by obtaining the particle size distributionon the volume basis and taking the whole to be 100%. The particle sizedistribution is obtained from the frequency distribution andaccumulative volume distribution curve measured by means of a laserscattering particle size distribution measuring apparatus. Themeasurement of particle sizes is carried out by sufficiently dispersingthe powder in an aqueous medium by e.g. an ultrasonic treatment andmeasuring the particle size distribution, for example, by means of alaser diffraction/scattering type particle size distribution measuringapparatus (trade name: Partica LA-950VII), manufactured by HORIBA, Ltd.

In the present invention, the average particle size (D50) of thelithium-containing composite oxide is preferably from 3 to 30 μm, morepreferably from 4 to 25 μm, particularly preferably from 5 to 20 μm.

The specific surface area of the lithium-containing composite oxide inthe present invention is preferably from 0.1 to 10 m²/g, particularlypreferably from 0.15 to 5 m²/g. When the specific surface area of thelithium-containing composite oxide is from 0.1 to m²/g, it is possibleto form a dense cathode layer having a high discharge capacity. Here,the specific surface area is a value measured by means of a nitrogen gasadsorption BET (Brunauer, Emmett, Teller) method.

In a case where the lithium-containing composite oxide is the compound(i) or the compound (iii), the specific surface area is preferably from0.1 to 3 m²/g, more preferably from 0.2 to 2 m²/g, particularlypreferably from 0.3 to 1 m²/g. In a case where the lithium-containingcomposite oxide is the compound (ii), the specific surface area ispreferably from 1 to 10 m²/g, more preferably from 2 to 8 m²/g,particularly preferably from 3 to 6 m²/g.

A method for producing the lithium-containing composite oxide may, forexample, be a method wherein a lithium compound and a precursor for alithium-containing composite oxide obtained by a coprecipitation method,are mixed and fired, a hydrothermal synthesis method, a sol-gel method,a dry blending method (solid phase method), an ion exchange method or aglass crystallization method. However, preferred is a method wherein alithium compound and a precursor for a lithium-containing compositeoxide obtained by a coprecipitation method (coprecipitated composition)are mixed and fired, whereby transition metal elements will be uniformlycontained, so that the discharge capacity will be excellent.

As the coprecipitation method, specifically an alkali coprecipitationmethod and a carbonate coporecipitation method are preferred. In thisspecification, the alkali coprecipitation method is a method wherein atransition metal salt aqueous solution and a pH adjusting agentcontaining a strong alkali are continuously added to a reaction solutionto form a transition metal hydroxide. The carbonate coprecipitationmethod is a method wherein a transition metal salt aqueous solution anda carbonate aqueous solution are continuously added to a reactionsolution to form a transition metal carbonate.

In the present invention, in the first embodiment, it is preferred touse a lithium-containing composite oxide produced from a precursorobtainable by the alkali coprecipitation method. From the precursorobtainable by the alkali coprecipitation method, a cathode materialwhich has a high powder density and may be highly filled in a battery,can be obtained. In the present invention, in the second embodiment, itis preferred to use a lithium-containing composite oxide produced from aprecursor obtainable by a carbonate coprecipitation method. From theprecursor obtainable by a carbonate coprecipitation method, a cathodematerial which is porous and has a high specific surface area and whichalso has a very high discharge capacity, can be obtained.

In the alkali coprecipitation method, the pH of the reaction solution ispreferably from 10 to 12. The pH adjusting agent to be added, ispreferably an aqueous solution containing, as a strong alkali, at leastone compound selected from the group consisting of sodium hydroxide,potassium hydroxide and lithium hydroxide. Further, an aqueous ammoniasolution or an aqueous ammonium sulfate solution may, for example, beadded to the reaction solution.

In the carbonate coprecipitation method, the pH of the reaction solutionis preferably from 7 to 9. The aqueous carbonate solution is preferablyan aqueous solution containing at least one compound selected from thegroup consisting of sodium carbonate, sodium hydrogencarbonate,potassium carbonate and potassium hydrogencarbonate. Further, an aqueousammonia solution or an aqueous ammonium sulfate solution may, forexample, be added to the reaction solution.

<Covering Layer>

The covering layer in the present invention is a layer formed on thesurface of particles of the lithium-containing composite oxide andcomprises a metal oxide (I) having at least one metal element selectedfrom the group consisting of elements in Groups 3 and 13 of the periodictable and lanthanoid elements, and a compound (II) containing Li and P.And, the atomic ratio of elements (P/metal element) contained within 5nm of the surface layer of the covering layer in the particles (III) isfrom 0.03 to 0.45.

Here, “covering” means a state chemically or physically adsorbed on apart or whole of the surface of the lithium-containing composite oxideparticles, and a layer so “covering” is referred to as a “coveringlayer”.

(Metal Oxide (I))

The metal oxide (I) in the present invention contains at least one metalelement selected from the group consisting of elements in Groups 3 and13 of the periodic table and lanthanoid elements. The metal elements inGroup 3 are Sc and Y, and the metal elements in Group 13 are Al, Ga, Inand Tl. The lanthanoid elements may be La, Pr, Nd, Gd, Dy, Er and Yb. Solong as the metal element is one selected from the group consisting ofelements in Groups 3 and 13 of the periodic table and lanthanoidelements, it is possible to form an electrochemically stable trivalentoxide coating film.

The metal element in the metal oxide (I) is preferably at least onemetal element selected from the group consisting of Al, Y, Ga, In, La,Pr, Nd, Gd, Dy, Er and Yb, more preferably at least one metal elementselected from the group consisting of Al, Y, Ga, La, Gd and Er,particularly preferably at least one metal element selected from thegroup consisting of Al and Y.

Specifically, the metal oxide (I) may, for example, be Al₂O₃, Y₂O₃,Ga₂O₃, In₂O₃, La₂O₃, Pr₂O₃, Nd₂O₃, Gd₂O₃, Dy₂O₃, Er₂O₃ or Yb₂O₃. Amongthem, Al₂O₃, Y₂O₃, Gd₂O₃ or Er₂O₃ is preferred, and Al₂O₃ isparticularly preferred, whereby the after-described discharge capacity,rate characteristics and cycle characteristics of the lithium ionsecondary battery will be excellent. The metal oxide (I) in the presentinvention may contain one or more of such metal oxides.

(Compound (II))

The compound (II) in the present invention contains Li and P.

The compound (II) is preferably Li₃PO₄, Li₄P₂O₇ or Li₃PO₃, morepreferably Li₃PO₄ since it is chemically most stable. The compound (II)may contain one or more of such compounds.

The metal oxide (I) in the covering layer may be crystalline oramorphous, preferably amorphous. Here, amorphous means that no peakattributable to the metal oxide (I) in the covering layer is observed inthe XRD measurement. The reason for the preference is not clear, but isconsidered to be such that when the metal oxide (I) is amorphous, themetal oxide (I) tends to easily elute to the electrolytic solution,whereby it functions as a sacrifice layer. That is, it is consideredthat, as the metal oxide (I) elutes to the electrolytic solution,elution to the electrolytic solution, of the transition metal elementsuch as Mn in the surface of the lithium-containing composite oxide willbe suppressed, whereby the cycle characteristics will be improved.

On the other hand, the compound (II) containing Li and P, in thecovering layer, is preferably crystalline. The compound (II) beingcrystalline means that in the XRD measurement, a peak attributable tothe compound (II) in the covering layer is observed. The reason for thepreference is not clear, but is considered to be such that thecrystalline compound tends to bring about higher mobility of lithiumions, whereby lithium diffusivity along with charge and discharge willbe improved, and the charge and discharge efficiency and the ratecharacteristics will be improved.

As exemplified in the after-described production process, the compound(II) containing Li and P in the covering layer, is formed by contactingan aqueous solution containing P as an anion to the lithium-containingcomposite oxide, followed by thermal treatment. As Li in the compound(II), it is possible to use lithium in the lithium-containing compositeoxide or a lithium compound such as lithium carbonate contained in avery small amount in the lithium-containing composite oxide. Byconsuming excess lithium in the lithium-containing composite oxide orthe lithium compound, it is possible to lower the alkali which causesgeneration of a gas.

Further, the covering layer may be one formed by collection of fineparticles chemically or physically adsorbed. In a case where thecovering layer is to be formed by collection of fine particles, theaverage particle size of the fine particles is preferably from 0.1 to100 nm, more preferably from 0.1 to 50 nm, particularly preferably from0.1 to 30 nm. Here, the average particle size is represented by anaverage value of diameters of fine particles which cover the surface ofthe lithium-containing composite oxide particles. The shape of thecovering layer and the average particle size of the fine particles canbe measured and evaluated by an electron microscope such as SEM(scanning electron microscope) or TEM (transmission electronmicroscope).

In the present invention, by such a covering layer, it is consideredpossible to reduce the contact of the lithium-containing composite oxideand the electrolytic solution, whereby it is possible to suppresselution of a transition metal such as Mn to the electrolytic solutionfrom the surface of the lithium-containing composite oxide and therebyto improve the cycle characteristics. Further, by the covering layer, itis considered possible to suppress deposition of decomposition productsof the electrolytic solution on the surface of the lithium-containingcomposite oxide, whereby it is possible to improve the ratecharacteristics.

<Particles (III) as Cathode Active Material>

The cathode active material for a lithium ion secondary battery of thepresent invention is particles (III) having a structure wherein thesurface of the lithium-containing composite oxide particles is coveredby the covering layer.

The shape of the particles (III) may be any of a spherical-form, afilm-form, a fiber-form, an agglomerated form, etc. In a case where theparticles (III) is in a spherical-form, the average particle size of theparticles (III) is preferably from 3 to 30 μm, more preferably from 4 to25 μm, particularly preferably from 5 to 20 μm.

The covering layer may cover at least a part of the surface of thelithium-containing composite oxide particles. Further, the particles(III) are preferably particles in which a part or whole of the surfaceof the lithium-containing composite oxide particles is covered by thecovering layer made of a mixture of amorphous and crystalline materials.Particularly preferred is a mixture wherein the metal oxide (I) isamorphous and the compound (II) is crystalline.

In the particles (III), the state where the covering layer is formed onthe surface of the lithium-containing composite oxide particles, can beevaluated, for example, by polishing the cross-section after cutting theparticles (III) and carrying out element mapping by an X-raymicroanalyzer method (ERMA). By this evaluation method, it is possibleto confirm that the metal element and P contained in the covering layerare present in a larger amount within a range of up to 30 nm from thesurface of the particles (III) than at the center of the particles(III). Here “the center of the particles (III)” is meant for a pointwhere the average distance from the surface of the particles (III) islongest.

In the particles (III), the content (molar amount) of the metal elementsin the covering layer as calculated from the feeding amount of the rawmaterial, is preferably a proportion of from 0.001 to 0.03, morepreferably a proportion of from 0.005 to 0.02, particularly preferably aproportion of from 0.01 to 0.015, to the molar amount of thelithium-containing composite oxide. When the content of the metalelements in the covering layer is from 0.001 to 0.03, a cathode activematerial having a large discharge capacity and being excellent in therate characteristics and cycle characteristics, can be obtained.

In the particles (III), the content (molar amount) of P in the coveringlayer as calculated from the feeding amount of the raw material, ispreferably a proportion of from 0.001 to 0.03, to the molar amount ofthe lithium-containing composite oxide. The content of P in the coveringlayer is more preferably from 0.005 to 0.025, particularly preferablyfrom 0.01 to 0.02.

The amounts (molar amounts) of metal elements and P present in thecovering layer of the particles (III) can be measured by dissolving theparticles (III) as a cathode active material in an acid and conductingan ICP (high frequency inductively-coupled plasma) measurement. Here, ina case where it is not possible to obtain the amounts of metals presentand P in the covering layer by the ICP measurement, they may becalculated based on e.g. the amounts of metal elements and P in theaqueous solution at the time of the production as described hereinafter.

The atomic ratio of elements (P/metal element) contained within 5 nm ofthe surface layer of the particles (III) is from 0.03 to 0.45. Thisatomic ratio is preferably from 0.05 to 0.45, further preferably from0.10 to 0.40, particularly preferably from 0.15 to 0.35, whereby thecompound (II) having P not contributing to development of the capacity,is small in amount, and excellent rate characteristics are obtainable.

In the present invention, the atomic ratio (P/metal element) within 5 nmof the surface layer of the particles (III) can easily be analyzed by anXPS (X-ray photoelectron spectroscopic) analysis. By using the XPSanalysis, it is possible to analyze the types of elements or theproportions of elements contained in a layer very close to the surfaceof the particles. Here, an example of the XPS analyzer may be ESCA Model5500 manufactured by PHI.

At the time of calculating an atomic ratio by using the XPS analysis inthe present invention, it is preferred to use, for the calculation,peaks which can be detected with a high sensitivity and which do notoverlap with peaks of other elements as far as possible. Specifically,at the time of analyzing Al and P, it is preferred to use 2P peaks, andat the time of analyzing Y, it is preferred to use 3d peaks.

In the particles (III) in the present invention, it is preferred that Pin the compound (II) has a concentration gradient such that itsconcentration gradually decreases from the surface of the particles(III) towards the center. It is considered that in the particles (III),P in the covering layer diffuses to the inside of the lithium-containingcomposite oxide, whereby the mobility of lithium is improved so thatintroduction or removal of lithium is facilitated.

The state where P has a concentration gradient in the particles (III),can be confirmed, for example, by the above-mentioned XPS analysis whilecarrying out etching by e.g. argon ions.

If a lithium compound such as lithium hydroxide or lithium carbonate(hereinafter referred to as a “free alkali”) is present in excess at thesurface of the particles (III) of the present invention, a decompositionreaction of the electrolytic solution is accelerated, thus causinggeneration of a gas of decomposition products. The amount of such a freealkali can be quantified as an amount which elutes when the cathodeactive material is dispersed in water. The amount of the free alkali inthe particles (III) of the present invention is preferably at most 2.0mol %, more preferably from 0 to 1.5 mol %.

In the cathode active material for a lithium ion secondary battery ofthe present invention, the covering layer comprises the metal oxide (I)and the compound (II), and the atomic ratio of elements (P/metalelement) contained within 5 nm of the surface layer, is adjusted to bewithin the specific range, whereby the discharge capacity, ratecharacteristics and cycle characteristics are improved. The reason forsuch improvements is not clearly understood, but is considered to besuch that the compound (II) in the covering layer is a compound havingan ionic bond, whereby as compared with e.g. a covering layer composedsolely of a metal oxide or the like wherein no ionically-bonded compoundis present, the mobility of lithium ions is improved, and the batterycharacteristics are improved. Further, the compound (II) is formed bywithdrawing lithium in the lithium-containing composite oxide, wherebythere will be no excessive alkali such as lithium present, and it ispossible to suppress formation of a gas due to decomposition of asolvent in the electrolytic solution.

Further, the covering layer contains the metal oxide (I), whereby it ispossible to form an oxide film which is electrochemically stable, and ina case where e.g. LiPF₆ is used as the after-described electrolyte, HFto be formed by decomposition of LiPF₆ can be reacted with and consumedby the metal oxide (I) in the covering layer. Accordingly, the cyclecharacteristics will be improved. Here, a stable oxide film means acompound having a strong bonding property to oxygen and can be comparedby Gibbs free energy values. In general, rather than a bivalent metaloxide, a trivalent metal oxide has a smaller Gibbs free energy value(negatively larger) and is more stable.

Further, it is considered that the rate characteristics are improved,since it is possible to suppress deposition of decomposition products ofthe electrolytic solution on the surface of the lithium-containingcomposite oxide. Furthermore, by adjusting the atomic ratio (P/metalelement) to be within the specific range, it is possible to improve therate characteristics and cycle characteristics without lowering thedischarge capacity.

The process for producing the cathode active material for a lithium ionsecondary battery of the present invention is not particularly limited,and for example, it can be produced by the following process.

[Process for Producing Cathode Active Material for Lithium Ion SecondaryBattery]

The process for producing a cathode active material for a lithium ionsecondary battery of the present invention comprises a first contactstep of contacting a powder of a lithium-containing composite oxidecomprising lithium and a transition metal element, and a first aqueoussolution which contains a cation having at least one metal elementselected from the group consisting of elements in Groups 3 and 13 of theperiodic table and lanthanoid elements, a second contact step ofcontacting said powder of the lithium-containing composite oxide, and asecond aqueous solution which contains an anion having P and which doesnot contain the cation of the metal element, and a heating step ofheating, after the first and second contact steps, the powder of thelithium-containing composite oxide to a temperature of from 250 to 700°C. And, the process is characterized in that in the entire aqueoussolution having the first and second aqueous solutions put together,|(number of moles of said anion contained in said second aqueoussolution×valency of said anion)|/(number of moles of said cationcontained in said first aqueous solution×valency of said cation) is lessthan 1.

In this specification, a “powder” means a collection of individualparticles. That is, in the first or second contact step of the presentinvention, the first aqueous solution or the second aqueous solution iscontacted with a powder made of a collection of the lithium-containingcomposite oxide particles.

By such a production process, it is possible to form a covering layercomprising the metal oxide (I) and the compound (II), on the surface ofthe particles of the lithium-containing composite oxide. And, it ispossible to produce, with good productivity, the cathode active materialfor a lithium ion secondary battery excellent in the cyclecharacteristics and rate characteristics even when charging is carriedout at a high voltage.

Now, the respective steps will be described.

<First Contact Step and Second Contact Step>

The first contact step is to contact a powder of the lithium-containingcomposite oxide and a first aqueous solution which contains a cationhaving at least one metal element selected from the group consisting ofelements in Groups 3 and 13 of the periodic table and lanthanoidelements. The second contact step is to contact said powder of thelithium-containing composite oxide, and a second aqueous solution whichcontains an anion having P and which does not contain the cation of themetal element. In each contact step, it is preferred to add the aqueoussolution to the powder of the lithium-containing composite oxide toobtain a wet powder.

Here, as will be mentioned later, the first contact step and the secondcontact step are preferably separate steps, but may be the same step.That is, the first aqueous solution containing a cation having the metalelement and the second aqueous solution containing an anion having saidP may be contacted with the lithium-containing composite oxidesimultaneously.

(Lithium-Containing Composite Oxide)

As the lithium-containing composite oxide, the above-describedlithium-containing composite oxide may be used, and the preferredembodiment may also be the same.

(First Aqueous Solution and Second Aqueous Solution)

The first aqueous solution to be used in the first contact step containsa cation having at least one metal element selected from the groupconsisting of elements in Groups 3 and 13 of the periodic table andlanthanoid elements.

The cation is preferably Al³⁺, V³⁺, Ga³⁺, In³⁺, La³⁺, Pr³⁺, Nd³⁺, Gd³⁺,Dy³⁺, Er³⁺ or Y³⁺, more preferably Al³⁺, Y³⁺, Ga³⁺, La³⁺, Gd³⁺ or Er³⁺.Further, the cation may be a complex ion having the above metal element,but it is preferably an ion of the above metal element from theviewpoint of the reactivity with the after-described anion. As thecation, Al³⁺ or Y³⁺ is particularly preferred in that a stable coatingfilm can be formed, the molecular weight of the cation is small, and thedischarge capacity per unit mass of the after-described lithium ionsecondary battery, rate characteristics and cycle characteristics willbe excellent.

Further, the first aqueous solution may contain cations which will bedecomposed and evaporated by heating, such as H⁺, NH₄ ⁺, etc, inaddition to the cation having the above metal element. Here, in thisspecification, “will be decomposed and evaporated by heating” means thatwhen heated to from 250 to 700° C. in the after-described heating step,the object will be decomposed and evaporated and will not remain in thecovering layer.

The second aqueous solution to be used in the second contact stepcontains an anion having P without containing a cation having the abovemetal element. Such an anion may, for example, be PO₄ ³⁻, P₂O₇ ⁴⁻, PO₃³⁻ or PO₂ ³⁻. Each of such anions will not be decomposed or evaporatedby heating and will become PO₄ ³⁻ in a stable oxide state.

Further, the second aqueous solution may contain anions which will bedecomposed and evaporated by heating, such as OH⁻, NO³⁻, CO₃ ²⁻, etc. inaddition to the anion having the above P.

The first aqueous solution can be obtained by dissolving a water-solublecompound containing the above metal element (hereinafter referred to asthe first water-soluble compound) in distilled water or the like, as asolvent (the solvent will be described later). Further, the secondaqueous solution can be obtained by dissolving a water-soluble compoundcontaining the above P (hereinafter referred to as the secondwater-soluble compound) in distilled water or the like, as a solvent.

Here, the “water-soluble” in the above water-soluble compound means thatthe solubility (the mass [g] of a solute dissolved in 100 g of asaturated solution) in distilled water at 25° C. is more than 2. Whenthe solubility is more than 2, it is possible to increase the amount ofthe water-soluble compound in the aqueous solution, whereby it ispossible to efficiently form a covering layer. The solubility of thewater-soluble compound is more preferably more than 5, particularlypreferably more than 10.

The first water-soluble compound containing the above metal element ispreferably a compound having the above metal element combined with ananion which will be decomposed and evaporated by heating, and forexample, an inorganic salt such as a nitrate, sulfate or chloride, anorganic salt such as an acetate, citrate, maleate, formate, lactate oroxalate, an organic complex, or an amine complex, of the above metalelement, may be mentioned. Among them, a nitrate, an organic acid salt,an organic complex or an amine complex is particularly preferred in thatthe solubility in a solvent is high, and the anion tends to be readilydecomposed by heat.

Specifically, the first water-soluble compound is preferably aluminumnitrate, aluminum acetate, aluminum oxalate, aluminum citrate, aluminumlactate, basic aluminum lactate, aluminum maleate, yttrium nitrate,yttrium formate, yttrium citrate, yttrium acetate or yttrium oxalate.

The second water-soluble compound containing P and containing an anionwhich will remain without being decomposed or evaporated by hearing, ispreferably a compound having the above anion combined with a cationwhich will be decomposed and evaporated by heating, and for example,acids such as H₃PO₄, H₄P₂O₇, H₃PO₃, H₃PO₂, etc., or their ammonium saltsor amine salts may be mentioned. Among them, the above ammonium saltsare particularly preferred, whereby the pH will not be low.

Specifically, the second water-soluble compound is more preferably(NH₄)₃PO₄, (NH₄)₂HPO₄ or (NH₄)H₂PO₄.

In the present invention, in the entire aqueous solution having thefirst and second aqueous solutions put together, in order to bring theabsolute value of the total of values obtained by multiplying the numberof moles by the valency of said anion containing P in the second aqueoussolution, to be small as compared with the total of values obtained bymultiplying the number of moles by the valency of said cation having themetal element in the first aqueous solution, it is preferred to use thefirst water-soluble compound having the metal element and having ananion which will be decomposed and evaporated by heating, and the secondwater-soluble compound having said P and having a cation which will bedecomposed and evaporated by heating, in combination.

Further, in the present invention, it is preferred to adjust the amount(molar amount) of the metal element contained in the covering layer tobe in a ratio of from 0.001 to 0.03 to the molar amount of thelithium-containing composite oxide. And, in order to control the amountof the metal element in the covering layer within this range, it ispreferred to adjust the amount (molar ratio) of the cation having themetal element contained in the first aqueous solution to be in a ratioof from 0.001 to 0.03 to the amount (molar amount) of thelithium-containing composite oxide. The value of the molar ratio of thecation to the lithium-containing composite oxide is more preferably from0.005 to 0.025, particularly preferably from 0.01 to 0.02.

Further, in order to control the amount of said P contained in thecovering layer, it is preferred to adjust the amount (molar amount) ofthe anion contained in the second aqueous solution to be in a ratio offrom 0.001 to 0.03 to the amount (molar amount) of thelithium-containing composite oxide. The value of the molar ratio of theanion to the lithium-containing composite oxide is more preferably from0.005 to 0.025, particularly preferably from 0.01 to 0.02.

The amount (molar amount) of the cation contained in the first aqueoussolution can be measured by conducting e.g. the above-mentioned ICP.Further, the amount (molar amount) of the anion contained in the secondaqueous solution can be measured by e.g. the above-mentioned ICP or ionchromatography.

Further, in the process of the present invention, in the entire aqueoussolution having the first and second aqueous solutions put together,{|number of moles of said anion×valency of said anion|/(number of molesof said cation×valency of said cation): hereinafter referred to as (Z)}is less than 1. Here, “| |” represents an absolute value. That is,although the valency of an anion becomes a negative value, the value of(Z) is made to be a positive number by taking the absolute value of(number of moles of the anion×valency of the anion). The value of (Z) iswithin a range of preferably from 0.1 to 0.8, more preferably from 0.2to 0.8, particularly preferably from 0.3 to 0.5.

By adjusting the amounts of the cation and the anion contained in thefirst aqueous solution and in the second aqueous solution to bring thevalue of (Z) to be less than 1, it is possible to adjust the atomicratio of elements (P/metal element) contained within 5 nm of the surfacelayer of the particles (III) to be within the range of from 0.03 to0.45.

For example, in a case where 1 mol % (0.01 mol) of Al³⁺ per 1 mol of thelithium ion composite oxide is incorporated in the covering layer,(number of moles of Al³⁺×valency of Al³⁺) becomes to be:0.01 mol×(+3)=0.03.

Further, for example, in a case where 0.5 mol % (0.005 mol) of PO₄ ³⁻per mol of the lithium ion composite oxide is incorporated in thecovering layer, the value of (number of moles of PO₄ ³⁻× valency of PO₄³⁻) becomes to be:|0.005 mol×(−3)|=0.015.

In the case of a combination of Al³⁺ and PO₄ ³⁻, (number of moles ofAl³⁺×valency of Al³⁺) as the cation which will remain without beingdecomposed or evaporated by heating is 0.03, and 0.015 being theabsolute value of (number of moles of PO₄ ³⁻×valency of PO₄ ³⁻) as theanion which will remain without being decomposed or evaporated byheating is smaller. And, the value of (Z) becomes to be 0.015/0.03=0.50.

In the present invention, as the solvent for the first and secondaqueous solutions, water such as distilled water may be used, and tosuch an extent not to impair the solubility of the water-solublecompound, a water-soluble alcohol or polyol may be added as a solvent.The water-soluble alcohol may, for example, be methanol, ethanol,1-propanol or 2-propanol. The polyol may, for example, be ethyleneglycol, propylene glycol, diethylene glycol, dipropylene glycol,polyethylene glycol, butanediol or glycerine.

The total content of the water-soluble alcohol and polyol is preferablyfrom 0 to 20 mass %, more preferably from 0 to 10 mass %, to the entireamount of the solvent. The solvent is preferably composed solely ofwater, as being excellent from the viewpoint of safety, environmentalaspect, handling efficiency and costs.

Further, at least one of the first aqueous solution and the secondaqueous solution may contain a pH adjusting agent to adjust thesolubility of the water-soluble compound. As such a pH adjusting agent,one which will be volatilized or decomposed when heated, is preferred.Specifically, an organic acid such as acetic acid, citric acid, lacticacid, formic acid, maleic acid or oxalic acid, or ammonia is preferred.When such a volatile or decomposable pH adjusting agent is used,impurities are less likely to remain, and good battery characteristicstend to be readily obtainable.

The pH of the first aqueous solution and the second aqueous solution ispreferably from 2 to 12, more preferably from 3 to 11, particularlypreferably from 4 to 10. When the pH is within such a range, at the timeof contacting the lithium-containing composite oxide with such anaqueous solution, elution of lithium or a transition metal from thelithium-containing composite oxide will be less, and impurities due toaddition of a pH adjusting agent or the like can be reduced, wherebygood battery characteristics tend to be readily obtainable.

In the first contact step and the second contact step in the presentinvention, the method for contacting the lithium-containing compositeoxide and the aqueous solution may, for example, be a method of adding aprescribed aqueous solution to a powder of the lithium-containingcomposite oxide, followed by stirring and mixing, or a method ofapplying a prescribed aqueous solution to a powder of thelithium-containing composite oxide by spray coating. The method ofapplying the aqueous solution by spray coating is more preferred, sincea filtration or washing step as a post-treatment step is not required,the productivity is excellent, and it is possible to form a coveringlayer uniformly on the surface of the lithium-containing composite oxideparticles.

Here, a “prescribed aqueous solution” is meant for the first aqueoussolution in the first contact step, and for the second aqueous solutionin the second contact step. The same applies in the followingdescription.

In the method for stirring and mixing, it is preferred that whilestirring the powder of the lithium-containing composite oxide, theprescribed aqueous solution is added thereto, and the prescribedwater-soluble compound contained in the aqueous solution is contacted tothe surface of the powder of the lithium-containing composite oxide. Asthe stirring apparatus, it is possible to use a stirring machine havinga low shearing force, such as a drum mixer or a solid air mixer. Bycontacting the prescribed aqueous solution and a powder of thelithium-containing composite oxide while stirring and mixing, it ispossible to obtain particles having a covering layer formed moreuniformly on the surface of the lithium-containing composite oxideparticles.

Here, the “prescribed water-soluble compound” is meant for the firstwater-soluble compound in the first aqueous solution, and for the secondwater-soluble compound in the second aqueous solution.

In the present invention, the first contact step and the second contactstep may be conducted simultaneously, and an aqueous solution containingboth of the cation and the anion may be contacted to the powder of thelithium-containing composite oxide. Otherwise, the first contact stepand the second contact step may be separate steps, and the first aqueoussolution containing the cation and the second aqueous solutioncontaining the anion may be separately contacted to the powder of thelithium-containing composite oxide.

In a case where the first aqueous solution and the second aqueoussolution are separately contacted to the lithium-containing compositeoxide, the order of contact may be such that after contacting the firstaqueous solution, the second aqueous solution may be contacted, or aftercontacting the second aqueous solution, the first aqueous solution maybe contacted. Otherwise, the first aqueous solution and the secondaqueous solution may be contacted alternately a plurality of times, ormay be contacted simultaneously. As the reaction of the cation with theanion is considered to readily proceed, it is particularly preferredthat the order is such that the second contact step is followed by thefirst contact step, i.e. after contacting the second aqueous solutioncontaining the anion to the power of the lithium-containing compositeoxide, the first aqueous solution containing the cation is contacted.

The concentration of the prescribed water-soluble compound in the firstaqueous solution or in the second aqueous solution is preferably high,since it is required to remove a solvent by heating as a post treatmentstep. However, if the concentration is too high, the viscosity tends tobe high, and the uniform mixing property of the lithium-containingcomposite oxide and the aqueous solution tends to deteriorate.Therefore, the concentration of the prescribed water-soluble compoundcontained in the aqueous solution is preferably from 0.5 to 30 mass %,more preferably from 2 to 20 mass %, as calculated based on elementconcentrations.

In the first contact step and the second contact step of the presentinvention, after the powder of the lithium-containing composite oxide iscontacted with the prescribed aqueous solution, drying may be conducted.In a case where spray coating is employed as the contacting method, thespray coating and the drying may be conducted alternately, or whileconducting spray coating, the drying may be simultaneously carried out.The drying temperature is preferably from 40 to 200° C., more preferablyfrom 60 to 150° C.

In a case where the lithium-containing composite oxide becomesagglomerates by the contact with the prescribed aqueous solution,followed by the drying, it is preferred to pulverize the agglomerates.The spraying amount of the aqueous solution in the spray coating ispreferably from 0.005 to 0.1 g/min. to 1 g of the lithium-containingcomposite oxide.

<Heating Step>

In the process of the present invention, after conducting theabove-described first contact step and second contact step, heating iscarried out. By the heating, it is possible to obtain the desiredcathode active material and at the same time to remove volatileimpurities such as water, organic components, etc.

The heating is preferably carried out in an oxygen-containingatmosphere. Further, the heating temperature is preferably from 250 to700° C., more preferably from 350 to 600° C. When the heatingtemperature is at least 250° C., it is possible to readily form acovering layer comprising a metal oxide (I) containing at least onemetal element selected from the group consisting of elements in Groups 3and 13 of the periodic table and lanthanoid elements, and a compound(II) containing Li and P. Further, volatile impurities such as residualmoisture will be less, whereby it is possible to suppress deteriorationof the cycle characteristics. Further, when the heating temperature isat most 700° C., it is possible prevent diffusion of the metal elementto the inside of the lithium-containing composite oxide and thus toprevent the covering layer from failing to function properly.

In order to form the metal oxide (I) to be amorphous on the surface ofthe lithium-containing composite oxide particles, the heatingtemperature is preferably from 250 to 550° C., more preferably from 350to 500° C. When the heating temperature is at most 550° C., the metaloxide (I) tends to be hardly crystallized.

The heating time is preferably from 0.1 to 24 hours, more preferablyfrom 0.5 to 18 hours, particularly preferably from 1 to 12 hours. Byadjusting the heating time within such a range, it is possible toefficiently form the covering layer on the surface of thelithium-containing composite oxide particles.

The pressure at the time of the heating is not particularly limited, andordinary pressure or elevated pressure is preferred, and ordinarypressure is particularly preferred.

The cathode active material obtainable by the process of the presentinvention is particles (III) having a covering layer comprising a metaloxide (I) containing at least one metal element selected from the groupconsisting of elements in Groups 3 and 13 of the periodic table andlanthanoid elements, and a compound (II) containing Li and P, on thesurface of particles of the lithium-containing composite oxide. And, thecovering layer is one to be formed by the first aqueous solution and thesecond aqueous solution which are used in the process of the presentinvention.

The details of the covering layer are as described in the above sectionfor cathode active material for lithium ion secondary battery of thepresent invention.

It is considered that in the cathode active material obtainable by theprocess of the present invention, contact of the lithium-containingcomposite oxide and the electrolytic solution is reduced by the coveringlayer, whereby elution of a transition metal such as Mn into theelectrolytic solution from the surface of the lithium-containingcomposite oxide is suppressed, and the cycle characteristics are therebyimproved. Further, it is considered that it is possible to suppressdeposition of decomposition products of the electrolytic solution, onthe surface of the lithium-containing composite oxide, whereby the ratecharacteristics are improved.

That is, as a result of coexistence of the compound (II) containing Liand P and the metal oxide (I) having the metal element in the coveringlayer, for example when LiPF₆ is used as an electrolyte, HF formed bydecomposition of LiPF₆ will be reacted with the metal oxide (I) andthus, HF will be consumed, whereby the cycle characteristics will beimproved.

Further, by adjusting the value of the above (Z) to be less than 1, thedischarge capacity, rate characteristics and cycle characteristics areimproved. The reason for the improvement is not clearly understood, butis considered to be such that by the presence of the compound (II)having an ion binding property in a proper amount, the mobility oflithium ions is improved, whereby the battery characteristics areimproved.

Still further, in the cathode active material obtainable by the processof the present invention, it is possible to prevent the amount of freealkali such as lithium hydroxide or lithium carbonate from becomingexcessive at the surface of the cathode active material, whereby it ispossible to suppress formation of a gas of decomposition products of theelectrolytic solution and to improve the battery characteristics.

[Cathode for Lithium Ion Secondary Battery]

The cathode for a lithium ion secondary battery of the present inventionhas a cathode active material layer comprising the cathode activematerial for a lithium ion secondary battery of the present invention,an electrically conductive material and a binder, formed on a cathodecurrent collector (cathode surface).

A method for producing such a cathode for a lithium ion secondarybattery may, for example, be a method wherein the above cathode activematerial, an electrically conductive material and a binder are supportedon a cathode current collector plate. At that time, the cathode can beproduced, for example, in such a manner that the electrically conductivematerial and the binder may be dispersed in a solvent and/or dispersingmedium to prepare a slurry, or kneaded with a solvent and/or dispersingmedium, to prepare a kneaded product, and the prepared slurry or kneadedproduct is supported on a cathode current collector plate by e.g.coating.

The electrically conductive material may, for example, be a carbon blacksuch as acetylene black, graphite or Ketjenblack.

The binder may, for example, be a fluoro resin such as polyvinylidenefluoride or polytetrafluoroethylene, a polyolefin such as polyethyleneor polypropylene, an unsaturated bond-containing polymer or copolymersuch as styrene/butadiene rubber, isoprene rubber or butadiene rubber,or an acrylic acid type polymer or copolymer such as an acrylic acidcopolymer or methacrylic acid copolymer.

The cathode current collector may, for example, be an aluminum foil oran aluminum alloy foil.

[Lithium Ion Secondary Battery]

The lithium ion secondary battery of the present invention comprises theabove-described cathode for a lithium ion secondary battery of thepresent invention, an anode and a non-aqueous electrolyte.

The anode comprises an anode current collector and an anode activematerial layer containing an anode active material, formed thereon. Theanode can be produced, for example, in such a manner that an anodeactive material is kneaded with an organic solvent to prepare a slurry,and the prepared slurry is applied to an anode current collector,followed by drying and pressing.

As the anode current collector, a metal foil such as a nickel foil orcupper foil may, for example, be used.

The anode active material may be any material so long as it is capableof absorbing and desorbing lithium ions at a relatively low potential.For example, it is possible to employ a lithium metal, a lithium alloy,a carbon material, an oxide composed mainly of a metal in Group 14 or 15of the periodic table, a carbon compound, a silicon carbide compound, asilicon oxide compound, titanium sulfide, a boron carbide compound, etc.

As the carbon material as the anode active material, it is possible touse, for example, non-graphitizable carbon, artificial graphite, naturalgraphite, thermally decomposed carbon, cokes such as pitch coke, needlecoke, petroleum coke, etc., graphites, glassy carbons, an organicpolymer compound fired product obtained by firing and carbonizing aphenol resin, furan resin, etc. at a suitable temperature, carbonfibers, activated carbon, carbon blacks, etc.

The metal in Group 14 of the periodic table may, for example, be siliconor tin, and most preferred is silicon.

Further, as a material which can be used as the anode active material,it is possible to use, for example, an oxide such as iron oxide,ruthenium oxide, molybdenum oxide, tungsten oxide, titanium oxide, tinoxide, etc. or a nitride such as Li_(2.6)Co_(0.4)N.

As the non-aqueous electrolyte solution, it is possible to use oneprepared by suitably combining an organic solvent and an electrolyte. Asthe organic solvent, it is possible to use a conventional one known asan organic solvent for an electrolytic solution, and for example, it ispossible to use propylene carbonate, ethylene carbonate, diethylcarbonate, dimethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane,diglyme, triglyme, γ-butyrolacton, diethyl ether, sulfolan, methylsulfolan, acetonitrile, an acetic acid ester, a butyric acid ester, apropionic acid ester, etc. Particularly, from the viewpoint of thevoltage stability, it is preferred to use a cyclic carbonate such aspropylene carbonate, or a chain-structured carbonate such as dimethylcarbonate or diethyl carbonate. Further, such organic solvents may beused alone, or two or more of them may be used as mixed.

Further, as a non-aqueous electrolyte, it is possible to use a solidelectrolyte containing an electrolyte salt, a polymer electrolyte, asolid or geled electrolyte having an electrolyte mixed or dissolved ine.g. a polymer compound, etc.

The solid electrolyte may be any material so long as it has lithium ionconductivity, and for example, either one of an inorganic solidelectrolyte and a polymer electrolyte may be used.

As the inorganic solid electrolyte, it is possible to use lithiumnitride, lithium iodide, etc.

As the polymer electrolyte, it is possible to use an electrolyte saltand a polymer compound which dissolves the electrolyte salt. And, assuch a polymer compound, it is possible to use polyethylene oxide,polypropylene oxide, polyphosphazene, polyaziridine, polyethylenesulfide, polyvinyl alcohol, polyvinylidene fluoride andpolyhexafluoropropylene, or their derivatives, mixtures and complexes.

The geled electrolyte may be any one so long as it is geled uponabsorption of the above non-aqueous electrolyte, and various polymersmay be employed. Further, as the polymer material to be used for thegeled electrolyte, it is possible to use, for example, a fluorinatedpolymer such as poly(vinylidene fluoride) or poly(vinylidenefluoride-co-hexafluoropropylene). Further, as a polymer material to beused for the geled electrolyte, it is possible to use, for example,polyacrylonitrile or a copolymer of polyacrylonitrile, as well as anether type polymer, such as a polyethylene oxide, or a copolymer orcross-linked product of polyethylene oxide. The copolymerizable monomermay, for example, be polypropylene oxide, methyl methacrylate, butylmethacrylate, methyl acrylate or butyl acrylate.

As the matrix of the geled electrolyte, a fluorinated polymer isparticularly preferred from the viewpoint of the stability against theredox reaction.

As the electrolyte salt, any one of those commonly used for batteries ofthis type may be used. For example, LiClO₄, LiPF₆, LiBF₄, CF₃SO₃Li,LiCl, LiBr, etc. may be used.

The shape of the lithium ion secondary battery of the present inventionmay be suitably selected depending on the intended use from e.g. acoin-shape, a sheet-form (film-form), a folded shape, a wound cylinderwith bottom, a button shape, etc.

According to the process for producing a cathode active material for alithium ion secondary battery of the present invention, it is possibleto produce with good productivity, a cathode active material for alithium ion secondary battery excellent in the cycle characteristics andrate characteristics even when charging is conducted at a high voltage.Further, according to the process of the present invention, a filtrationor washing step as a post-treatment step is not required, thelithium-containing composite oxide will not be agglomerated, thehandling such as stirring is easy, and no agglomeration is likely tooccur during the drying, whereby the productivity will be remarkablyimproved.

Further, according to the cathode for a lithium ion secondary battery ofthe present invention employing the cathode active material obtainableby the process of the present invention, and the lithium ion secondarybattery employing the cathode, it is possible to realize excellent cyclecharacteristics and rate characteristics, even when charging isconducted at a high voltage.

EXAMPLES

Now, the present invention will be described in detail with reference toExamples, but it should be understood that the present invention is byno means restricted by the Examples.

Synthesis Example for Lithium-Containing Composite Oxide (A)

By adding 1,245.9 g of distilled water to a mixture of 140.6 g ofnickel(II) sulfate hexahydrate, 131.4 g cobalt(II) sulfate heptahydrateand 482.2 g of manganese(II) sulfate pentahydrate, a raw materialsolution was obtained wherein the above compounds were uniformlydissolved. Further, by adding 320.8 g of distilled water, 79.2 g ofammonium sulfate was uniformly dissolved to obtain an ammonium sulfatesolution. By adding 1,920.8 g of distilled water, 79.2 g of ammoniumsulfate was uniformly dissolved to obtain a mother liquid. By adding 600g of distilled water, 400 g of sodium hydroxide was uniformly dissolvedto obtain a pH-adjusting liquid.

Into a 2 L baffle-equipped glass reactor, the mother liquid was put andheated to 50° C. by a mantle heater, and the pH-adjusting liquid wasadded to bring the pH to be 11.0. Then, while stirring the solution inthe reactor by anchor-type stirring vanes, the raw material solution wasadded at a rate of 5.0 g/min, and the ammonium sulfate solution wasadded at a rate of 1.0 g/min, to have a composite hydroxide of nickel,cobalt and manganese precipitated. During the addition of the rawmaterial solution, the pH-adjusting solution was added to maintain thepH in the reactor to be 11.0. Further, in order to prevent oxidation ofthe precipitated hydroxide, nitrogen gas was introduced into the reactorat a flow rate of 0.5 L/min. Further, the liquid was withdrawncontinuously so that the amount of the liquid in the reactor would notexceed 2 L.

In order to remove impurity ions from the obtained composite hydroxideof nickel, cobalt and manganese, pressure filtration and dispersion todistilled water were repeated for washing. The washing was terminatedwhen the electrical conductivity of the filtrate became less than 25μS/cm, followed by drying at 120° C. for 15 hours to obtain a precursor.

The contents of nickel, cobalt and manganese in the precursor weremeasured by ICP and found to be 11.6 mass %, 10.5 mass % and 42.3 mass%, respectively. The molar ratio of nickel:cobalt:manganese was found tobe 0.172:0.156:0.672.

Then, 20 g of this precursor and 12.6 g of lithium carbonate having alithium content of 26.9 mol/kg were mixed and fired at 900° C. for 12hours in an oxygen-containing atmosphere to obtain a powder of alithium-containing composite oxide. This powder will be referred to aslithium-containing composite oxide (A).

The composition of the obtained lithium-containing composite oxide (A)was Li (Li_(0.2)Ni_(0.137)Co_(0.125)Mn_(0.538))O₂. Thelithium-containing composite oxide (A) had an average particle size D50of 5.9 μm, and a specific surface area of 2.6 m²/g as measured by meansof a nitrogen adsorption BET method.

Synthesis Example for Lithium-Containing Composite Oxide (B)

By adding 1,245 g of distilled water to a mixture of 197 g of nickel(II)sulfate hexahydrate, 105 g cobalt(II) sulfate heptahydrate and 452 g ofmanganese(II) sulfate pentahydrate, a raw material solution was obtainedwherein the above compounds were uniformly dissolved. Further, by adding401 g of distilled water, 99 g of ammonium sulfate was uniformlydissolved to obtain an ammonium sulfate solution. By adding 1,900 g ofdistilled water, 1 g of sodium carbonate was uniformly dissolved toobtain a mother liquid. Further, by adding 1,850 g of distilled water,350 g of sodium carbonate was uniformly dissolved to obtain an aqueouscarbonate solution.

Then, into a 2 L baffle-equipped glass reactor, the mother liquid wasput and heated to 50° C. by a mantle heater. While stirring the solutionin the reactor by double stage inclined paddle type stirring blades, theraw material solution was added at a rate of 5.0 g/min, and the ammoniumsulfate solution was added at a rate of 0.5 g/min, over a period of 6hours, to have a composite carbonate of nickel, cobalt and manganeseprecipitated. Here, during the addition of the raw material solution,the aqueous carbonate solution was added to maintain the pH in thereactor to be 8.0. Further, in order to prevent oxidation of theprecipitated transition metal carbonate, nitrogen gas was introducedinto the reactor at a flow rate of 0.5 L/min.

In order to remove impurity ions from the obtained composite carbonateof nickel, cobalt and manganese, pressure filtration and dispersion todistilled water were repeated for washing. The washing was terminatedwhen the electrical conductivity of the filtrate became less than 100μS/cm, followed by drying at 120° C. for 15 hours to obtain a precursor.

The contents of nickel, cobalt and manganese in the precursor weremeasured by ICR, whereby the molar ratio of nickel:cobalt:manganese wasfound to be 0.245:0.126:0.629. Further, the content of a transitionmetal contained in the precursor was determined by back titration byZINCON indicator, EDTA and an aqueous zinc chloride solution and foundto be 8.23 mol/kg.

Then, 20 g of this precursor and 8.2 g of lithium carbonate having alithium content of 26.9 mol/kg were mixed and fired at 850° C. for 16hours in an oxygen-containing atmosphere to obtain a powder of alithium-containing composite oxide. This powder will be referred to aslithium-containing composite oxide (B).

The composition of the obtained lithium-containing composite oxide (B)was Li (Li_(0.143)Ni_(0.210)Co_(0.108)Mn_(0.539))O₂. Thelithium-containing composite oxide (B) had an average particle size D50of 11.2 μm, and a specific surface area of 6.8 m²/g as measured by meansof a nitrogen adsorption BET method.

Synthesis Example for Lithium-Containing Composite Oxide (C)

A precursor was obtained in the same manner as in Synthesis Example forlithium-containing composite oxide (B), except that a raw materialsolution obtained by adding 1,253 g of distilled water to a mixture of260 g of nickel(II) sulfate hexahydrate, 17 g cobalt(II) sulfateheptahydrate and 470 g of manganese(II) sulfate pentahydrate, and havingthe above compounds uniformly dissolved therein, was used as the rawmaterial solution.

The contents of nickel, cobalt and manganese in the precursor weremeasured by ICR, whereby the molar ratio of nickel:cobalt:manganese wasfound to be 0.326:0.020:0.654. Further, the content of a transitionmetal contained in the precursor was determined by back titration byZINCON indicator, EDTA and an aqueous zinc chloride solution and foundto be 8.53 mol/kg.

Then, 20 g of this precursor and 8.25 g of lithium carbonate having alithium content of 26.9 mol/kg were mixed and fired at 850° C. for 16hours in an oxygen-containing atmosphere to obtain a powder of alithium-containing composite oxide. This powder will be referred to aslithium-containing composite oxide (C).

The composition of the obtained lithium-containing composite oxide (C)was Li (Li_(0.130)Ni_(0.283)Co_(0.017)Mn_(0.569))O₂. Thelithium-containing composite oxide (C) had an average particle size D50of 11.2 μm, and a specific surface area of 9.2 m²/g as measured by meansof a nitrogen adsorption BET method.

Example 1

To 7.0 g of a raw material aluminum lactate aqueous solution having analuminum content of 4.5 mass % and a pH of 4.6, 3.0 g of distilled waterwas added and mixed to prepare an aqueous aluminum lactate solution.Further, to 0.77 g of ammonium hydrogenphosphate ((NH₄)₂HPO₄), 9.23 g ofdistilled water was added and mixed to prepare an aqueous ammoniumhydrogenphosphate solution.

Then, while stirring the lithium-containing composite oxide (A) obtainedas described above, to 10 g thereof, 1 g of the aqueous ammoniumhydrogenphosphate solution was sprayed by a spray coating method, andthe lithium-containing composite oxide (A) and the aqueous ammoniumhydrogenphosphate solution were mixed and contacted. Then, 1 g of theaqueous aluminum lactate solution prepared as described above wassprayed by a spray coating method, and the lithium-containing compositeoxide (A) and the aqueous aluminum lactate solution were mixed andcontacted, to obtain a mixture. Here, the value of (Z) of the cation(Al³⁺) and the anion (PO₄ ³⁻) sprayed on the lithium-containingcomposite oxide (A) was 0.50.

Then, the obtained mixture was dried at 90° C. for 2 hours and thenheated at 400° C. for 8 hours in an oxygen-containing atmosphere toobtain a cathode active material (1) made of particles (III) having acovering layer containing Al and P on the surface of thelithium-containing composite oxide particles.

In the cathode active material (1) thus obtained, the value of the molarratio of Al contained in the covering layer by the aqueous aluminumlactate solution to the lithium-containing composite oxide is calculatedby {(number of moles of Al in covering layer (I))/(number of moles oflithium-containing composite oxide)} and was 0.01.

Then, with respect to the obtained cathode active material (1), an XRDmeasurement was carried out under the after-described conditions. Fromthe measured XRD spectrum, the cathode active material (1) was confirmedto have a layered rock salt type crystal structure (space group R-3m).Further, from FIG. 1, a peak of layered Li₂MnO₃ was observed within arange of 2θ=20 to 25° and further, a peak attributable to Li₃PO₄ as thecompound (II) containing P was observed. On the other hand, in the XRDspectrum, no peak attributable to the metal oxide (I) containing Al wasobserved. Compounds having peaks detected by XRD, are shown in Table 1.

Then, with respect to the obtained cathode active material (1), an XPSmeasurement was carried out under the after-described conditions. UsingAl₂O₃, AlPO₄ and Li₃PO₄ as comparative samples, chemical shifts ofAl_(2p) and P_(2p) of the cathode active material (1) were compared. Asa result, as shown in FIGS. 2 and 3, the chemical shifts of Al_(2p) andP_(2p) of the cathode active material (1) were found to agree to thechemical shifts of Al₂O₃ and Li₃PO₄, respectively. Further, from theresults of this measurement, the atomic ratio (P_(2p)/Al_(2p)) of P tothe metal element (Al) was calculated. These results of the XPSmeasurement are shown in Table 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe compound containing the metal element (Al) contained in the coveringlayer was Al₂O₃, and the compound (II) containing P was Li₃PO₄. Further,Al₂O₃ was not detected by XRD and therefore is considered to beamorphous.

Further, with respect to the obtained cathode active material (1),measurement of the free alkali amount was conducted by theafter-described method. The result is shown in Table 1.

Example 2

An aqueous ammonium hydrogenphosphate solution was prepared by adding8.77 g of distilled water to 1.23 g of ammonium hydrogenphosphate((NH₄)₂HPO₄). And, in the same manner as in Example 1 except that thisaqueous ammonium hydrogenphosphate solution was sprayed and contacted tothe lithium-containing composite oxide (A), a cathode active material(2) was obtained, which was made of particles (III) having a coveringlayer containing Al and P on the surface of the lithium-containingcomposite oxide particles.

Here, the value of (Z) of the cation (Al³⁺) and the anion (PO₄ ³⁻)sprayed on the lithium-containing composite oxide (A) was 0.80.

In the cathode active material (2) thus obtained, the value of the molarratio of Al contained in the covering layer by the aqueous aluminumlactate solution to the lithium-containing composite oxide is calculatedby {(number of moles of Al in covering layer (I))/(number of moles oflithium-containing composite oxide)} and was 0.01.

Then, with respect to the obtained cathode active material (2), an XRDmeasurement, an XPS measurement and a measurement of a free alkaliamount were carried out in the same manner as in Example 1. The XRDspectrum is shown in Table 1, and the XPS spectra are shown in Tables 2and 3. Further, compounds having their peaks detected by the XRDspectrum and the result of measurement of the free alkali amount areshown in Table 1, and the results of the XPS measurement are shown inTable 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe cathode active material (2) had a layered rock salt type crystalstructure (space group R-3m), a peak of layered Li₂MnO₃ was observedwithin a range of 2θ=20 to 25°, the compound containing the metalelement (Al) contained in the covering layer was Al₂O₃, and the compound(II) containing P was Li₃PO₄. Further, Al₂O₃ was not detected by XRD andtherefore is considered to be amorphous.

Example 3

An aqueous ammonium hydrogenphosphate solution was prepared by adding9.54 g of distilled water to 0.46 g of ammonium hydrogenphosphate((NH₄)₂HPO₄). And, in the same manner as in Example 1 except that thisaqueous ammonium hydrogenphosphate solution was sprayed and contacted tothe lithium-containing composite oxide (A), a cathode active material(3) was obtained, which was made of particles (III) having a coveringlayer containing Al and P on the surface of the lithium-containingcomposite oxide particles.

Here, the value of (Z) of the cation (Al³⁺) and the anion (PO₄ ³⁻)sprayed on the lithium-containing composite oxide (A) was 0.30.

In the cathode active material (3) thus obtained, the value of the molarratio of Al contained in the covering layer by the aqueous aluminumlactate solution to the lithium-containing composite oxide is calculatedby {(number of moles of Al in covering layer (I))/(number of moles oflithium-containing composite oxide)} and was 0.01.

Then, with respect to the obtained cathode active material (3), an XRDmeasurement, an XPS measurement and a measurement of a free alkaliamount were carried out in the same manner as in Example 1. Compoundshaving their peaks detected by the XRD spectrum and the result ofmeasurement of the free alkali amount are shown in Table 1, and theresults of the XPS measurement are shown in Table 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe cathode active material (3) had a layered rock salt type crystalstructure (space group R-3m), a peak of layered Li₂MnO₃ was observedwithin a range of 2θ=20 to 25°, the compound containing the metalelement (Al) contained in the covering layer was Al₂O₃, and the compound(II) containing P was Li₃PO₄. Further, Al₂O₃ was not detected by XRD andtherefore is considered to be amorphous.

Comparative Example 1

The lithium-containing composite oxide (A) obtained as described abovewas used as a cathode active material (4) of Comparative Example 1 as itwas i.e. without subjecting it to covering treatment (formation of acovering layer).

With respect to the obtained cathode active material (4), an XRDmeasurement and a measurement of the free alkali amount were carried outin the same manner as in Example 1. From the measured XRD spectrum, thecathode active material (4) was confirmed to have a layered rock salttype crystal structure (space group R-3m). Further, a peak of layeredLi₂MnO₃ was observed within a range of 2θ=20 to 25°. Compounds havingthe peaks detected by the XRD spectrum and the result of measurement ofthe free alkali amount are shown in Table 1.

Comparative Example 2

In Example 1, without spraying an aqueous ammonium hydrogenphosphatesolution, only 1 g of the aqueous aluminum lactate solution was sprayedby a spray coating method to the lithium-containing composite oxide (A).Otherwise, in the same manner as in Example 1, a cathode active material(5) was obtained, which was made of particles (III) having a coveringlayer containing Al on the surface of the lithium-containing compositeoxide particles.

In the cathode active material (5) thus obtained, the value of the molarratio of Al contained in the covering layer by the aqueous aluminumlactate solution to the lithium-containing composite oxide is calculatedby {(number of moles of Al in covering layer (I))/(number of moles oflithium-containing composite oxide)} and was 0.01.

Then, with respect to the obtained cathode active material (5), an XRDmeasurement, an XPS measurement and a measurement of a free alkaliamount were carried out in the same manner as in Example 1. Compoundshaving their peaks detected by the XRD spectrum (shown in FIG. 1) andthe result of measurement of the free alkali amount are shown in Table1, and the results of the XPS measurement are shown in Table 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe cathode active material (5) had a layered rock salt type crystalstructure (space group R-3m), a peak of layered Li₂MnO₃ was observedwithin a range of 2θ=20 to 25°, and the compound containing the metalelement (Al) contained in the covering layer was Al₂O₃. Further, Al₂O₃was not detected by XRD and therefore is considered to be amorphous.

Comparative Example 3

An aqueous ammonium hydrogenphosphate solution was prepared by adding8.46 g of distilled water to 1.54 g of ammonium hydrogenphosphate((NH₄)₂HPO₄). And, in the same manner as in Example 1 except that thisaqueous ammonium hydrogenphosphate solution was sprayed and contacted tothe lithium-containing composite oxide (A), a cathode active material(6) was obtained, which was made of particles (III) having a coveringlayer containing Al and P on the surface of the lithium-containingcomposite oxide particles.

Here, the value of (Z) of the cation (Al³⁺) and the anion (PO₄ ³⁻)sprayed on the lithium-containing composite oxide (A) was 1.00.

In the cathode active material (6) thus obtained, the value of the molarratio of Al contained in the covering layer by the aqueous aluminumlactate solution to the lithium-containing composite oxide is calculatedby {(number of moles of Al in covering layer (I))/(number of moles oflithium-containing composite oxide)} and was 0.01.

Then, with respect to the obtained cathode active material (6), an XRDmeasurement, an XPS measurement and a measurement of a free alkaliamount were carried out in the same manner as in Example 1. Compoundshaving their peaks detected by the XRD spectrum and the result ofmeasurement of the free alkali amount are shown in Table 1, and theresults of the XPS measurement are shown in Table 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe cathode active material (6) had a layered rock salt type crystalstructure (space group R-3m), a peak of layered Li₂MnO₃ was observedwithin a range of 2θ=20 to 25°, the compound containing the metalelement (Al) contained in the covering layer was Al₂O₃, and the compound(II) containing P was Li₃PO₄. Further, Al₂O₃ was not detected by XRD andtherefore is considered to be amorphous.

Comparative Example 4

An aqueous ammonium hydrogenphosphate solution was prepared by adding8.07 g of distilled water to 1.93 g of ammonium hydrogenphosphate((NH₄)₂HPO₄). And, in the same manner as in Example 1 except that thisaqueous ammonium hydrogenphosphate solution was sprayed and contacted tothe lithium-containing composite oxide (A), a cathode active material(7) was obtained, which was made of particles (III) having a coveringlayer containing Al and P on the surface of the lithium-containingcomposite oxide particles.

Here, the value of (Z) of the cation (Al³⁺) and the anion (PO₄ ³⁻)sprayed on the lithium-containing composite oxide (A) was 1.25.

In the cathode active material (7) thus obtained, the value of the molarratio of Al contained in the covering layer by the aqueous aluminumlactate solution to the lithium-containing composite oxide is calculatedby {(number of moles of Al in covering layer (I))/(number of moles oflithium-containing composite oxide)} and was 0.01.

Then, with respect to the obtained cathode active material (7), an XRDmeasurement, an XPS measurement and a measurement of a free alkaliamount were carried out in the same manner as in Example 1. Compoundshaving their peaks detected by the XRD spectrum and the result ofmeasurement of the free alkali amount are shown in Table 1, and theresults of the XPS measurement are shown in Table 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe cathode active material (7) had a layered rock salt type crystalstructure (space group R-3m), a peak of layered Li₂MnO₃ was observedwithin a range of 2θ=20 to 25°, the compound containing the metalelement (Al) contained in the covering layer was Al₂O₃, and the compound(II) containing P was Li₃PO₄. Further, Al₂O₃ was not detected by XRD andtherefore is considered to be amorphous.

Example 4

To 2.90 g of yttrium(III) nitrate hexahydrate, 7.10 g of distilled waterwas added and mixed to prepare an aqueous yttrium nitrate solution.Further, to 1.49 g of ammonium hydrogenphosphate ((NH₄)₂HPO₄), 8.51 g ofdistilled water was added and mixed to prepare an aqueous ammoniumhydrogenphosphate solution.

Then, while stirring the lithium-containing composite oxide (B) obtainedas described above, to 10 g thereof, 0.3 g of the aqueous ammoniumhydrogenphosphate solution prepared as described above was sprayed by aspray coating method, and the lithium-containing composite oxide (B) andthe aqueous ammonium hydrogenphosphate solution were mixed andcontacted. Then, 1.5 g of the aqueous yttrium nitrate solution preparedas described above was sprayed by a spray coating method, and thelithium-containing composite oxide (B) and the aqueous aluminum lactatesolution were mixed and contacted, to obtain a mixture. Here, the valueof (Z) of the cation (Y³⁺) and the anion (PO₄ ³⁻) sprayed on thelithium-containing composite oxide (B) was 0.3.

Then, the obtained mixture was dried at 90° C. for 2 hours and thenheated at 400° C. for 8 hours in an oxygen-containing atmosphere toobtain a cathode active material (8) made of particles (III) having acovering layer containing Y and P on the surface of thelithium-containing composite oxide particles.

In the cathode active material (8) thus obtained, the value of the molarratio of Y contained in the covering layer by the aqueous aluminumlactate solution to the lithium-containing composite oxide is calculatedby {(number of moles of Y in covering layer (I))/(number of moles oflithium-containing composite oxide)} and was 0.01.

Then, with respect to the obtained cathode active material (8), an XRDmeasurement and a measurement of a free alkali amount were carried outin the same manner as in Example 1. Compounds having their peaksdetected by the XRD spectrum and the result of measurement of the freealkali amount are shown in Table 1. Then, with respect to the obtainedcathode active material (8), an XPS measurement was carried out. UsingLi₃PO₄ as a comparative sample, a chemical shift of P_(2p) was compared.From the peak of Y_(3d) and the peak of P_(2p), the atomic ratio(P_(2p)/Y_(3d)) of P to the metal element (Y) was calculated. Theseresults of the XPS measurement are shown in Table 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe cathode active material (8) had a layered rock salt type crystalstructure (space group R-3m), a peak of layered Li₂MnO₃ was observedwithin a range of 2θ=20 to 25°, and the compound (II) containing P wasLi₃PO₄. The compound containing a metal element (Y) contained in thecovering layer was not identified, but since no peak was detected byXRD, it is considered to be amorphous.

Example 5

To 9.02 g of a raw material aluminum lactate aqueous solution having analuminum content of 4.5 mass % and a pH of 4.6, 0.98 g of distilledwater was added and mixed to prepare an aqueous aluminum lactatesolution. Further, to 1.49 g of ammonium hydrogenphosphate ((NH₄)₂HPO₄),8.51 g of distilled water was added and mixed to prepare an aqueousammonium hydrogenphosphate solution.

Then, while stirring the lithium-containing composite oxide (C) obtainedas described above, to 10 g thereof, 0.3 g of the aqueous ammoniumhydrogenphosphate solution prepared as described above was sprayed by aspray coating method, and the lithium-containing composite oxide (C) andthe aqueous ammonium hydrogenphosphate solution were mixed andcontacted. Then, 1.5 g of the aqueous aluminum lactate solution preparedas described above was sprayed by a spray coating method, and thelithium-containing composite oxide (C) and the aqueous aluminum lactatesolution were mixed and contacted, to obtain a mixture. Here, the valueof (Z) of the cation (Al³⁺) and the anion (PO₄ ³⁻) sprayed on thelithium-containing composite oxide (C) was 0.15.

Then, the obtained mixture was dried at 90° C. for 2 hours and thenheated at 400° C. for 8 hours in an oxygen-containing atmosphere toobtain a cathode active material (9) made of particles (III) having acovering layer containing Al and P on the surface of thelithium-containing composite oxide particles.

In the cathode active material (9) thus obtained, the value of the molarratio of Al contained in the covering layer by the aqueous aluminumlactate solution to the lithium-containing composite oxide is calculatedby {(number of moles of Al in covering layer (I))/(number of moles oflithium-containing composite oxide)} and was 0.02.

Then, with respect to the obtained cathode active material (9), an XRDmeasurement, an XPS measurement and a measurement of a free alkaliamount were carried out in the same manner as in Example 1. Compoundshaving their peaks detected by the XRD spectrum and the result ofmeasurement of the free alkali amount are shown in Table 1, and theresults of the XPS measurement are shown in Table 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe cathode active material (9) had a layered rock salt type crystalstructure (space group R-3m), a peak of layered Li₂MnO₃ was observedwithin a range of 2θ=20 to 25°, the compound containing the metalelement (Al) contained in the covering layer was Al₂O₃, and the compound(II) containing P was Li₃PO₄. Further, Al₂O₃ was not detected by XRD,and therefore, is considered to be amorphous.

Example 6

In the same manner as in Example 5 except that the sprayed amount of theaqueous ammonium hydrogenphosphate was changed to 0.8 g, a cathodeactive material (10) was obtained which was made of particles (III)having a covering layer containing Al and P on the surface of thelithium-containing composite oxide particles.

Here, the value of (Z) of the cation (Al³⁺) and the anion (PO₄ ³⁻)sprayed on the lithium-containing composite oxide (A) was 0.50.

In the cathode active material (10) thus obtained, the value of themolar ratio of Al contained in the covering layer by the aqueousaluminum lactate solution to the lithium-containing composite oxide iscalculated by {(number of moles of Al in covering layer (I))/(number ofmoles of lithium-containing composite oxide)} and was 0.02.

Then, with respect to the obtained cathode active material (10), an XRDmeasurement, an XPS measurement and a measurement of a free alkaliamount were carried out in the same manner as in Example 1. Compoundshaving their peaks detected by the XRD spectrum and the result ofmeasurement of the free alkali amount are shown in Table 1, and theresults of the XPS measurement are shown in Table 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe cathode active material (10) had a layered rock salt type crystalstructure (space group R-3m), a peak of layered Li₂MnO₃ was observedwithin a range of 2θ=20 to 25°, the compound containing the metalelement (Al) contained in the covering layer was Al₂O₃, and the compound(II) containing P was Li₃PO₄. Further, Al₂O₃ was not detected by XRD,and therefore, is considered to be amorphous.

Comparative Example 5

In Example 4, without spraying an aqueous ammonium hydrogenphosphatesolution, only 1.5 g of the aqueous yttrium nitrate solution was sprayedby a spray coating method to the lithium-containing composite oxide (B).Otherwise, in the same manner as in Example 4, a cathode active material(11) was obtained, which was made of particles (III) having a coveringlayer containing Y on the surface of the lithium-containing compositeoxide particles.

In the cathode active material (11) thus obtained, the value of themolar ratio of Y contained in the covering layer by the aqueous yttriumnitrate solution to the lithium-containing composite oxide is calculatedby {(number of moles of Y in covering layer (I))/(number of moles oflithium-containing composite oxide)} and was 0.01.

Then, with respect to the obtained cathode active material (11), an XRDmeasurement, an XPS measurement and a measurement of a free alkaliamount were carried out in the same manner as in Example 4. Compoundshaving their peaks detected by the XRD spectrum and the result ofmeasurement of the free alkali amount are shown in Table 1, and theresults of the XPS measurement are shown in Table 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe cathode active material (11) had a layered rock salt type crystalstructure (space group R-3m), a peak of layered Li₂MnO₃ was observedwithin a range of 2θ=20 to 25°, and the compound (II) containing P wasLi₂PO₄. The compound containing the metal element (Y) contained in thecovering layer was not identified, but since no peak was detected byXRD, it is considered to be an amorphous compound.

Comparative Example 6

In the same manner as in Example 4 except that the sprayed amount of theaqueous ammonium hydrogenphosphate was changed to 1.2 g, a cathodeactive material (12) was obtained which was made of particles (III)having a covering layer containing Y and P on the surface of thelithium-containing composite oxide particles.

Here, the value of (Z) of the cation (Y³⁺) and the anion (PO₄ ³⁻)sprayed on the lithium-containing composite oxide (B) was 1.2.

In the cathode active material (12) thus obtained, the value of themolar ratio of Y contained in the covering layer by the aqueous yttriumnitrate solution to the lithium-containing composite oxide is calculatedby {(number of moles of Y in covering layer (I))/(number of moles oflithium-containing composite oxide)} and was 0.01.

Then, with respect to the obtained cathode active material (12), an XRDmeasurement and a measurement of a free alkali amount were carried outin the same manner as in Example 1. Compounds having their peaksdetected by the XRD spectrum and the result of measurement of the freealkali amount are shown in Table 1.

Then, with respect to the obtained cathode active material (12), an XPSmeasurement was carried out. Using Li₃PO₄ as a comparative sample, achemical shift of P_(2p) was compared. From the peak of Y_(3d) and thepeak of P_(2p), the atomic ratio (P_(2p)/Y_(3d)) of P to the metalelement (Y) was calculated. These results of the XPS measurement areshown in Table 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe cathode active material (12) had a layered rock salt type crystalstructure (space group R-3m), a peak of layered Li₂MnO₃ was observedwithin a range of 2θ=20 to 25°, and the compound (II) containing P wasLi₃PO₄. The compound containing a metal element (Y) contained in thecovering layer was not identified, but since no peak was detected byXRD, it is considered to be an amorphous compound.

Comparative Example 7

In Example 5, without spraying an aqueous ammonium hydrogenphosphatesolution, only 1.5 g of the aqueous aluminum lactate solution wassprayed by a spray coating method to the lithium-containing compositeoxide (C). Otherwise, in the same manner as in Example 5, a cathodeactive material (13) was obtained, which was made of particles (III)having a covering layer containing Al on the surface of thelithium-containing composite oxide particles.

In the cathode active material (13) thus obtained, the value of themolar ratio of Al contained in the covering layer by the aqueousaluminum lactate solution to the lithium-containing composite oxide iscalculated by {(number of moles of Al in covering layer (I))/(number ofmoles of lithium-containing composite oxide)} and was 0.02.

Then, with respect to the obtained cathode active material (13), an XRDmeasurement, an XPS measurement and a measurement of a free alkaliamount were carried out in the same manner as in Example 1. Compoundshaving their peaks detected by the XRD spectrum and the result ofmeasurement of the free alkali amount are shown in Table 1, and theresults of the XPS measurement are shown in Table 2.

From the results of the XRD and XPS measurements, it was confirmed thatthe cathode active material (13) had a layered rock salt type crystalstructure (space group R-3m), a peak of layered Li₂MnO₃ was observedwithin a range of 2θ=20 to 25°, and the compound containing the metalelement (Al) contained in the covering layer was Al₂O₃. Further, Al₂O₃was not detected by XRD and therefore is considered to be amorphous.

TABLE 1 Peak of metal Peak of Free Cathode element non-metal alkaliactive Metal compound compound amount material element P detecteddetected (mol %) Ex. 1 (1) Al Yes No Li₃PO₄ 1.18 Ex. 2 (2) Al Yes NoLi₃PO₄ 1.45 Ex. 3 (3) Al Yes No Li₃PO₄ 0.82 Comp. (4) No No No No 0.88Ex. 1 Comp. (5) Al No No No 0.47 Ex. 2 Comp. (6) Al Yes No Li₃PO₄ 1.75Ex. 3 Comp. (7) Al Yes No Li₃PO₄ 2.41 Ex. 4 Peak of metal Peak of FreeCathode element non-metal alkali active Metal compound compound amountEx. material element P detected detected (mol %) Ex. 4  (8) Y Yes NoLi₃PO₄ 0.62 Ex. 5  (9) Al Yes No Li₃PO₄ 1.16 Ex. 6 (10) Al Yes No Li₃PO₄1.47 Comp. (11) Y No No No 0.45 Ex. 5 Comp. (12) Y Yes No Li₃PO₄ 0.77Ex. 6 Comp. (13) Al No No No 1.20 Ex. 7

TABLE 2 Cathode Chemical Chemical Atomic ratio active Metal shift shift(P/metal Ex. material element P of Al_(2p) of P_(2p) element) Ex. 1 (1)Al Yes Agreed to Agreed to 0.27 Al₂O₃ Li₃PO₄ Ex. 2 (2) Al Yes Agreed toAgreed to 0.31 Al₂O₃ Li₃PO₄ Ex. 3 (3) Al Yes Agreed to Agreed to 0.15Al₂O₃ Li₃PO₄ Comp. (4) No No — — — Ex. 1 Comp. (5) Al No Agreed to — —Ex. 2 Al₂O₃ Comp. (6) Al Yes Agreed to Agreed to 0.49 Ex. 3 Al₂O₃ Li₃PO₄Comp. (7) Al Yes Agreed to Agreed to 0.65 Ex. 4 Al₂O₃ Li₃PO₄ Ex. 4 (8) YYes — Agreed to 0.35 Li₃PO₄ Ex. 5 (9) Al Yes Agreed to Agreed to 0.08Al₂O₃ Li₃PO₄ Ex. 6 (10)  Al Yes Agreed to Agreed to 0.41 Al₂O₃ Li₃PO₄Comp. (11)  Y No — — — Ex. 5 Comp. (12)  Y Yes — Agreed to 064 Ex. 6Li₃PO₄ Comp. (13)  Al No Agreed to — — Ex. 7 Al₂O₃

The specific surface areas of cathode active materials (1) to (13) areshown in Table 3.

TABLE 3 Specific surface area of Cathode cathode active material activematerial [m²/g] Ex. 1  (1) 2.4 Ex. 2  (2) 2.4 Ex. 3  (3) 2.4 Comp. Ex. 1 (4) 2.6 Comp. Ex. 2  (5) 2.4 Comp. Ex. 3  (6) 2.5 Comp. Ex. 4  (7) 2.8Ex. 4  (8) 6.0 Ex. 5  (9) 9.1 Ex. 6 (10) 9.2 Comp. Ex. 5 (11) 6.0 Comp.Ex. 6 (12) 6.4 Comp. Ex. 7 (13) 9.1<XRD Measurement>

For the XRD measurement, RINT-TTR-III, trade name, manufactured byRigaku Corporation, was used as an X-ray diffraction apparatus. As theX-ray source, CuKα ray was used. The measurement was conducted undersuch conditions that the voltage was 50 kV, the tube current was 300 mA,the scan axis was 2θ/θ, the measuring range was θ=10 to 90°, thesampling width was 0.04°, and the scanning speed was 1°/min, and thenunder such conditions that the measuring range was 2θ=20 to 36°, thesampling width was 0.04°, and the scanning speed was 0.27 min. Here, inFIG. 1 showing the results of the XRD measurement within a measuringrange of 2θ=20 to 36°, the base lines of the respective graphs are shownas spaced at a certain distance from one another in order to make iteasy to confirm the peaks of the respective graphs with respect to theresults of measurement in the respective Examples and ComparativeExamples.

<XPS Measurement>

A sample was prepared by densely transferring the cathode activematerial on a carbon tape. In the XPS measurement, using an X-rayphotoelectron spectroscopic apparatus Model 5500 manufactured by PHI(radiation source: AlKα, monochromatic), peaks on the low energy side ofC_(1s) are deemed to be contamination and adjusted at 284.8 eV. Themeasured area was within a circle with a diameter of about 800 μm. Themeasurement was conducted under such conditions that the wide scan pulseenergy was 93.9 eV and step energy was 0.8 eV, and the narrow scan(FIGS. 2 and 3) pulse energy was 23.5 eV and step energy was 0.05 eV.Here, in FIGS. 2 and 3 showing the results of the XPS measurement, thebase lines of the respective graphs are shown as spaced at a certaindistance from one another in order to make it easy to confirm the peaksof the respective graphs with respect to the results of measurement inthe respective Examples and for comparative samples.

<Free Alkali Measurement>

The measurement of the free alkali amount was carried out in such amanner that 1 g of a cathode material was dispersed in 50 g of purewater, followed by stirring for 30 minutes and then by filtrationthrough a membrane filter, whereupon the filtrate was titrated with a0.02 mol/L HCl aqueous solution. The total alkali amount was calculatedon the assumption that the titer to pH 8.5 corresponds to lithiumhydroxide (LiOH) and one lithium of lithium carbonate (Li₂CO₃) and thetiter from pH 8.5 to pH 4.0 corresponds to the remaining one lithium oflithium carbonate.

[Production of Cathode Sheet]

One of cathode active materials (1) to (7) obtained in Examples 1 to 3and Comparative Examples 1 to 4, acetylene black as electricallyconductive material and a solution (solvent: N-methylpyrrolidone)containing 12.1 mass % of polyvinylidene fluoride (binder), were mixed,and N-methylpyrrolidone was further added to prepare a slurry. At thattime, the mass ratio of the cathode active material, acetylene black andpolyvinylidene fluoride was made to be 82:10:8.

Then, the slurry was applied on one side of an aluminum foil (cathodecurrent collector) having a thickness of 20 μm by means of a doctorblade, followed by drying at 120° C. and then by roll pressing twice toprepare a cathode sheet. Here, cathode sheets obtained from the cathodeactive materials (1) to (3) in Examples 1 to 3 are designated as cathodesheets 1 to 3, respectively, and cathode sheets obtained from thecathode active materials (4) to (7) in Comparative Examples 1 to 4 aredesignated as cathode sheets 4 to 7, respectively. Further, cathodesheets obtained from the cathode active materials (8) to (10) inExamples 4 to 6 are designated as cathode sheets 8 to 10, respectively,and cathode sheets obtained from the cathode active materials (11) to(13) in Comparative Examples 5 to 7 are designated as cathode sheets 11to 13, respectively.

[Production of Lithium Ion Secondary Battery]

Using as a cathode one of the cathode sheets 1 to 13 obtained asdescribed above, a stainless steel simple sealed cell type lithium ionsecondary battery was assembled in an argon globe box. Here, a metallithium foil having a thickness of 500 μm was used as an anode, astainless steel plate having a thickness of 1 mm was used as an anodecurrent collector, and a porous polypropylene having a thickness of 25μm was used as a separator. Further, as an electrolytic solution, LiPF₆at a concentration of 1 mol/dm³/EC (ethylene carbonate)+DEC (diethylcarbonate) (1:1) solution (which means a mixed solution having LiPF₆ asa solute dissolved in EC and DEC in a volume ratio (EC:DEC=1:1)) wasused.

Here, lithium ion secondary batteries employing the cathode sheets 1 to13, respectively, are designated as batteries 1 to 13, respectively.

[Evaluations of Lithium Ion Secondary Batteries]

With respect to the batteries 1 to 13 produced as described above, thefollowing evaluations were carried out.

(Initial Capacity)

Charging to 4.7 V was conducted with a load current of 200 mA per 1 g ofthe cathode active material, and then discharging to 2.5 V was conductedwith a load current of 50 mA per 1 g of the cathode active material.Then, charging to 4.3 V was conducted with a load current of 200 mA per1 g of the cathode active material, and then discharging to 2.5 V wasconducted with a load current of 100 mA per 1 g of the cathode activematerial.

With respect to batteries 1 to 7 subjected to such charge and discharge,charging to 4.6 V was continuously conducted with a load current of 200mA per 1 g of the charge/discharge cathode active material, and thendischarging to 2.5 V was conducted with a load current of 100 mA per 1 gof the cathode active material. At that time, the discharge capacity ofthe cathode active material at from 4.6 to 2.5 V was taken as theinitial capacity at 46 V.

(Rate Characteristics)

After the evaluation of the initial capacity at 4.6 V, charging to 4.6 Vwas conducted with a load current of 200 mA per 1 g of thecharge/discharge cathode active material, and high rate discharging to2.5 V was conducted with a load current of 1,000 mA per 1 g of thecathode active material. At that time, the discharge capacity of thecathode active material at from 4.6 to 2.5 V in the high ratedischarging was divided by the initial capacity at 4.6 V to obtain avalue, and this value was taken as a rate retention.

(Cycle Characteristics)

A charge/discharge cycle of charging to 4.6 V with a load current of 200mA per 1 g of the charge/discharge cathode active material and high ratedischarging to 2.5 V with a load current of 100 mA per 1 g of thecathode active material, was repeated 50 times. At that time, thedischarge capacity in the 50th charge/discharge cycle at 4.6 V wasdivided by the initial capacity at 4.6 V to obtain a value, and thisvalue was taken as the cycle retention.

The results of evaluation of the above initial capacity at 4.6 V, rateretention and cycle retention with respect to the batteries 1 to 7, areshown in Table 4.

TABLE 4 Initial Cathode capacity Rate Cycle active Metal of 4.6 Vretention retention material element P (Z) (mAh/g) (%) (%) Battery 1 (1)Al Yes 0.50 227 76 95 Battery 2 (2) Al Yes 0.80 222 73 94 Battery 3 (3)Al Yes 0.30 225 75 92 Battery 4 (4) No No — 216 65 78 Battery 5 (5) AlNo — 205 63 84 Battery 6 (6) Al Yes 1.00 219 69 90 Battery 7 (7) Al Yes1.25 212 66 89

With respect to the batteries 8 to 13 produced as described above, thefollowing evaluations were carried out.

(Initial Capacity)

Charging to 4.6 V was conducted with a load current of 20 mA per 1 g ofthe cathode active material, and then discharging to 2.0 V was conductedwith a load current of 20 mA per 1 g of the cathode active material. Atthat time, the discharge capacity of the cathode active material at from4.6 to 2.0 V was taken as the initial capacity at 46 V. Further, thevalue of discharge capacity/charge capacity was calculated, and thisvalue was taken as the charge/discharge efficiency.

(Cycle Characteristics)

Then, a charge/discharge cycle of charging to 4.5 V with a load currentof 200 mA per 1 g of the cathode active material and high ratedischarging to 2.0 V with a load current of 200 mA per 1 g of thecathode active material, was repeated 100 times. At that time, thedischarge capacity in the first charge/discharge cycle at 4.5 V wastaken as the initial capacity at 4.5 V. Further, the discharge capacityin the 100th charge/discharge cycle at 4.5 V was divided by thedischarge capacity in the first charge/discharge cycle at 4.5 V toobtain a value, and this value was taken as the cycle retention.

The results of evaluation of the above initial capacity at 4.6 V,charge/discharge efficiency, initial capacity at 4.5 V and cycleretention with respect to the batteries 8 to 13, are shown in Table 5.

TABLE 5 Initial Charge/ Initial Cathode capacity discharge capacityCycle active Metal at 4.6 V efficiency at 4.5 V retention Ex. materialelement P (Z) (mAh/g) (%) (mAh/g) (%) Battery  (8) Y Yes 0.3 262 86 19689  8 Battery  (9) Al Yes 0.15 272 94 214 88  9 Battery (10) Al Yes 0.5272 95 214 88 10 Battery (11) Y No — 261 85 195 84 11 Battery (12) Y Yes1.2 265 88 192 70 12 Battery (13) Al No — 270 93 212 83 13

From Table 4, it is evident that, as compared with the lithium batteries4 to 7 employing the cathode active materials (4) to (7) in ComparativeExamples 1 to 4, the lithium batteries 1 to 3 employing the cathodeactive materials (1) to (3) in Examples 1 to 3 have high initialcapacities, are excellent in rate retention and yet have a high cycleretention of more than 90%.

As shown in Table 5, it is evident that the lithium battery 8 employingthe cathode active material (8) exhibits a high value in each of theinitial capacity at 4.6 V, charge/discharge efficiency, initial capacityat 4.5 V and cycle retention, as compared with the lithium battery 11employing the cathode active material (11) not containing P inComparative Example 5, and the lithium batteries 9 and 10 employing thecathode active materials (9) and (10) exhibit a high value in each ofthe initial capacity at 4.6 V, charge/discharge efficiency, initialcapacity at 4.5 V and cycle retention, as compared with the lithiumbattery 13 employing the cathode active material (13) not containing Pin Comparative Example 7. Further, it is evident that the lithiumbattery 12 employing the cathode active material (12) containing Pexcessively in Comparative Example 6, has a cycle retention lowered.

Accordingly it is evident that in a case where a cathode is prepared byusing the cathode active material for a lithium ion secondary battery ofthe present invention, and a lithium ion secondary battery isconstituted by using this cathode, it is possible to obtain a highinitial capacity and excellent rate retention and cycle retention.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to obtain a cathodeactive material for a lithium ion secondary battery, which has a highdischarge capacity per unit mass and which is excellent in cyclecharacteristics and rate characteristics. This cathode active materialis useful for a small sized light weight lithium ion secondary batteryfor electronic instruments such as cell phones or for vehicles.

What is claimed is:
 1. A cathode active material suitable for a lithiumion secondary battery, comprising: particles (III) comprising alithium-containing composite oxide; and a covering layer on a surface ofthe lithium-containing composite oxide, wherein the covering layercomprises: a metal oxide (I) comprising a metal element comprising aGroup 3, Group 13, and/or lanthanoid element; and a compound (II)comprising Li and P, wherein the metal oxide (I) is crystalline, whereinthe particles (III) have an atomic ratio of the P to the metal element,within 5 nm of the surface layer of the particles (III), in a range offrom 0.03 to 0.45, wherein the lithium-containing composite oxide has aformula (2-1):Li(Li_(x)Mn_(y)Me′_(z))O_(p)F_(q)  (2-1), wherein Me′ is Co, Ni, Cr, Fe,Al, Ti, Zr, and/or Mg, 0.09<x<0.3, y>0, z>0, 1.9<p<2.1, 0≤q≤0.1,0.4≤y/(y+z)≤0.8, x+y+z=1, and 1.2<(1+x)/(y+z), wherein in an X-raydiffraction measurement of the lithium-containing composite oxide usingCuKα ray as an X-ray source, a 2θ peak is observed within a range offrom 20 to 25°, and wherein the covering layer is configured to directlycontact the lithium-containing composite oxide and an electrolyticsolution.
 2. The material of claim 1, wherein the metal element is atleast one metal element selected from the group consisting of Al, Y, Ga,In, La, Pr, Nd, Gd, Dy, Er, and Yb.
 3. The material of claim 1, whereinthe compound (II) is Li₃PO₄.
 4. The material of claim 1, wherein theatomic ratio of the P to the metal element is in a range of from 0.10 to0.40.
 5. The material of claim 1, wherein the molar ratio of the metalelement to the lithium-containing composite oxide is in a range of from0.001 to 0.03.
 6. A cathode suitable for a lithium ion secondarybattery, the cathode comprising: the material of claim 1; and a binder.7. A lithium ion secondary battery, comprising: the cathode of claim 6;an anode; and a non-aqueous electrolyte.
 8. The material of claim 1,wherein the metal element in the covering layer is present in a largeramount within a range up to 30 nm from the surface of the particle (III)than at the center of the particle (III).
 9. The material of claim 1,wherein, in the lithium-containing composite oxide of formula (2-1),1.25≤(1+x)/(y+z)≤1.75 is satisfied.
 10. The material of claim 1, whereinthe lithium-containing composite oxide has a formula (2-2):Li(Li_(x)Mn_(y)Ni_(v)Co_(w))O_(p)  (2-2), wherein 0.09<x<0.3,0.36<y<0.73, 0<v<0.32, 0<w<0.32, 1.9<<2.1, and x+y+v+w=1.
 11. Thematerial of claim 1, wherein in the lithium-containing composite oxideof formula (2-1), 0.16≤x<0.3.
 12. The material of claim 1, wherein, inthe lithium-containing composite oxide of formula (2-1), 2≤p<2.1 andq=0.
 13. The material of claim 1, wherein the compound (II) iscrystalline.
 14. The material of claim 1, wherein the metal elementcomprises Y, Ga, In, La, Pr, Nd, Gd, Dy, Er, and/or Yb.
 15. The materialof claim 1, wherein the metal element comprises Al.
 16. The material ofclaim 1, wherein the metal element comprises Y.
 17. The material ofclaim 1, wherein the metal element comprises Gd.
 18. The material ofclaim 1, wherein the metal element comprises Er.
 19. The material ofclaim 1, wherein the particles (II) comprise only one covering layer.20. The material of claim 1, wherein the covering layer is formed by acollection of particles having an average particle size in the range offrom 0.1 to 100 nm.